user802: India is the original land of Buddhism, but it is followed/practiced more outside its country of birth. Are there any reasons for this?

user985: One of the main reasons for the decline of Buddhism in India is that it was engulfed by Hinduism in the years following the death of Buddha. 
India, before the time of Buddha was a Brahminical society dominated by Brahmins and Kshatriyas who were at the top of a four level caste system, the others being Vaishyas followed by Shudras. The Indian society was rife with rites and rituals which could be only performed by Brahmins. A householder throughout his existence was supposed to carry out various sacrifices which included animal slaughter and thus the vasihyas and the shudras suffered the most during pre-Buddha period. Now Buddha's teachings gave no value to rites and rituals and also he preached non-violence. This made Buddhism much popular among the vaishyas and shudras. This was of course a threat to Brahminical dominance and thus the livelihood of the priestly class. Hence there were many attempts by the priestly class to sabotage Buddhist teachings even during the time of Buddha but they were unsuccessful. 
After his death, they seem to have found success. Although Buddha negated the existence of God as is accepted in Hinduism, later Hindu fanatics made out Buddha himself to be a Hindu God. They declared him to be the 9th avatar of Vishnu, a Hindu God.

user3602: Brahmins were rarely involved in any wars (unless they were in the ruling position). The Vaishyas were never oppressed. In fact, they were wealthy land owners, money lenders etc. Hinduism is not a religion, its a culture. Hindus never oppressed any other religion, but have been oppressed. 
Taken from http://www.esamskriti.com/essay-chapters/Why-did-Buddhism-vanish-from-India-1.aspx :
I have listed down six reasons why Buddhism vanished from India. It is followed by the words of Swami Vivekananda & Dr B R Ambedkar.

The main cause was the neglect by the monks of this life and its values. While the Buddhist monks realized that everyone was not fit or could not become a monk or nun, they paid attention only to the life of a monk and not to the life of a householder. Which meant that they focused on the life of a monk, which is a life of inwardness as compared to that of a householder, which is one of outwardness. Now, both these aspects need examination, study, guidance and control. It is not enough to tell a householder that this existing life is only a stepping-stone to the life of a monk. Why and how is it so and what relation it bears to realities has to be explained. Instead Buddhist philosophers began to teach that this life was nothing but a value of tears and misery. While some forms of Vedanta taught the same philosophy, the attitude of Mimamsa (philosophy of action) and the Epics saved Hinduism from the fate that overtook Buddhism in India. Many great Indians were impacted by spiritual teachings but “unless there were some codes extolling the values of the world, they tended to become one-sidedly inwardly”.
Another reason was the admission of women into monasteries and the more or less indiscriminate conversion of men, women into monks and nuns. While true renunciation and celibacy were appreciated, people wanted to see them well practiced. When people supported these monasteries with their hard-earned money, they did not want its residents to live in luxury and enjoyment, virtues, which were condemned. If monks and nuns had lived by the rules that they were taught, people would have supported them inspite of any hardship that they had to face.
The next reason was the deterioration in the political and economic life of the country. Monasteries were supported by the people and the Kings e.g. Ashoka. Now, when a dynasty fell or a king died, the next in line might not give the same degree of support. The king’s thinkers realized that their defeat was due to the loss of their best fighters, leaders, who had become monks. This made the country an easy prey to the foreign invader. Coincidence or otherwise, India’s first foreign invasion by the Greeks took place in 327 B.C. a couple of centuries after Emperor Asoka’s peace movement.
Buddhism existed in the monasteries and unlike the dharmaasutras (ethical codes) lacked a moral code. So when monasteries disappeared, Buddhism disappeared. The invasion of the Muslims and the ruthless destruction of Buddhist monasteries extinguished the lamp of Buddhism in North India. The wanton destruction of the great monastery of Uddandapura (Bihar) and the wholesale massacre of its monks might make us visualize how the great monasteries of Nalanda, Vikramasila and others met with a tragic end.
The extreme asceticism practiced and popularized by both Buddhism and Jainism disturbed the social life of India. Magadha, the seat of many imperial dynasties, became Bihar, the land of monasteries (viharas). There was nothing in these religions to emphasize the importance of life in this world and its values. These causes led to a bloodless revolt by the orthodox in the eight-century a.d. The revolt was staged from two sides, the Brahmanic and the Upanisadic. Kumarila was the leader of the former and Sankara of the latter. Kumarila succeeded in reviving a strong positive attitude towards the world and its values and all that could be called human and activistic. On the other hand, Sankara said that everything that was good in Buddhism already existed in the Upanishads. In fact, Gaudapada, the grand teacher of Sankara, unified the current spanda (vibration) doctrine of Saivism, the vijnana (mind) doctrine of the Buddhists and the Atman doctrine of the Upanishads in his Mandukyakarikas and made the way easy for Sankara to assimilate and absorb Buddhism. Thus, there remained no justification for its separate existence in India; it had no social ethics and consequently, no hold over society. It could not stand alone as a spiritual discipline as it was shown to be part of the Upanishads.
Quoting Swami Vivekananda “ Thus, inspite of preaching mercy to animals, inspite of the sublime ethical religion, inspite of the discussions about the existence or non-existence of a permanent soul, the whole building of Buddhism tumbled down piece-meal and the ruin was simply hideous. The most hideous ceremonies, the most obscene books that human hands ever wrote or the human brain ever conceived, have all been the creation of the degraded Buddhism. The Tartars and the Baluchis and all the hideous races of mankind that came to India, became Buddhists and assimilated with us, brought their national customs and the whole of our national life became a huge page of the most horrible, bestial customs. Sankara came and showed that the real essence of Buddhism and that of Vedanta are not very different but that the disciples did not understand the master and have degraded themselves, denied the existence of soul and one God and have become atheists. That was what Sankara showed and all the Buddhists began to come back to their old religion”.
Buddhism adopted various thoughts and beliefs between the first century B.C. and the sixth century a.d. Some Buddhists adopted the tantric sadhanas and distorted them for the sake of enjoyment and comfort. The highly advanced philosophy of tantric sadhana is difficult to understand without the guidance of a proper teacher. This undigested knowledge of tantra, including the use of wine, meat, fish, gestures and physical union led these Buddhist followers to their downfall. Also, the distortions of Buddhism produced a variety of schools, which were not pure Buddhist schools but contained a variety of practices. To give you an idea of the syntheses between Vedanta and Buddhism, the concept of Maya in Vedanta in borrowed from Buddhism. Sankara accepted the logical connotation of Maya just as it was given by the Buddhists. Jainism was saved by tacitly allowing its members to become part of the Hindu fold by adopting rules of conduct of the third caste, namely Vaisyas or traders.
Quoted from ‘Dr Ambedkar Life & Mission by Dhananjay Keer’. Dr B R Ambedkar addressed delegates of Young Men’s Buddhist Association in May 1950 at Colombo on ‘Rise & fall of Buddhism in India’ - ‘Buddhism in its material force had disappeared. But as a spiritual force it still exists’. As regards Hinduism he said it went through three phases, Vedic religion, Brahmanism and Hinduism. It was during the Brahmanism period that Buddhism was born. It was not true that after the days of Shankaracharya Buddhism was dead in India. It was going on for years together. In fact Shankaracharya and his teacher were both Buddhists he added. While he was digging material on the subject for the decline/vanish of Buddhism from India the reasons were – adoption of some rituals & practices from Buddhism by the Vaishnava & Shaiva cults, which were vociferous in their propaganda against Buddhism. During the invasion by Allauddin Khilji thousands of priests in Bihar were massacred and consequently some of them fled for their lives to Tibet, China & Nepal. In the meanwhile, the majority of Buddhists went over to Hinduism. The third cause was that Buddhism was difficult to practice while Hinduism was not. Reason four was that the political atmosphere in India had been unfavorable to the advancement of Buddhism he concluded.

But according to Hindu scholars the fall of Buddhism was due to many reasons. Owing to universalistic ambition its spread was everywhere but it had geographical center nowhere. It discarded all national gods & godmen & proclaimed Buddha the greatest of all gods. As long as it reacted as a reformative flank in India, Buddhism gained ground but when it began to act against the Vedic religion, which was the national religion of the majority, Buddhism lost sympathy in India. The Vedic Hindus fought the Muslims bravely and did not flee to any other country. But the Buddhists when attacked, having a center nowhere, fled to different countries and even it is said acclaimed the invasion of India by non-Hindus with the ringing of bells. Besides its godlessness, its over-emphasis on redemption, its sad tone, its unconcern with the world & neglect of family checked rather than fostered enterprise. Quote ends. 
Books referred to 1. Introduction to Comparative Philosophy by P T Raju. 2. History & Culture of Indian People by Bhartiya Vidya Bhavan.

user471: Buddhism declined from India in the past, due to assimilation.
Firstly, Buddhism had flourished in ancient India, Pakistan and Afghanistan, but eventually, the Buddhist locals became subject to "hard" assimilation by foreign invaders. See the Wikipedia articles Buddhism in Afghanistan, Buddhism in Pakistan and also this article section. This is not unlike the disappearance of Zoroastrianism from its homeland of Persia.
Secondly, Buddhism suffered from "soft" assimilation into Hinduism.
Before Buddhism (around 500 BCE), the historic Vedic religion was mainly focused on sacrifices and rituals. It did not have robust philosophy and metaphysics. After encountering Buddhism and the shramanic movement (which includes Jainism), the Upanishadic movement rose in India from 500 - 200 BCE. After this, the Vedanta movement rose as part of Classical Hinduism from 200 BCE - 1100 CE. The Hindu text Brahma Sutras that attempted to refute Buddhist and other non-Hindu teachings, was written between 200 BCE and 400 CE. See this question for details.
Adi Shankara, the famous reformer who revived Hinduism and consolidated Advaita Vedanta, lived around 800 CE. Advaita is the philosophical school of Vedanta that is closest to Buddhism. Even in those days, Hindu philosophical opponents of Shankara accused of him of being a crypto-Buddhist. Through Advaita Vedanta, Hinduism assimilated Buddhism by adapting parts of its philosophy unto itself. Some Hindus even claim that Mahayana's emptiness (shunyata) is exactly the same as Advaita's Transcendental Ultimate Reality. See this answer for details.
On the other hand, the Dvaita Vedanta school, which has completely incompatible philosophy to Buddhism, assimilated Buddhism by making Buddha an incarnation of the Hindu deity Vishnu. The Hindu text Bhagavata Purana written between 500 CE and 1000 CE states that Vishnu incarnated as Buddha to delude the immoral atheists into living ethical lives. According to them, the Buddha tricked atheists into taking refuge in the Buddha and serving the Buddha, who is actually God in disguise. More info on this is available on this page.
Hindu philosophers also claimed that  Brahma Sutras, Bhagavata Purana and other such texts were divinely authored a long time before the arrival of the Buddha. This was one way to convince Buddhists that Vishnu's incarnation as the Buddha was foretold by the Hindu scriptures even before his birth. This was debunked by professional academicians.
In my personal opinion (which may lean towards Theravada), the final reason for Buddhism's "soft" assimilation into Hinduism is the development of concepts in Mahayana Buddhism that sound very similar to concepts in Hinduism, like the Eternal Buddha, Adi Buddha (Primordial Buddha), Buddha Nature (tathagatagarbha), storehouse consciousness (alayavijnana) and beings like Avalokiteshvara etc.
In his book "Indian Philosophy: A Critical Survey" on pages 63 - 64, Professor Chandradhar Sharma wrote very misleadingly about Buddhism:

Buddha is here transformed (by Mahayana Buddhism) into God and  worshipped  as 
  such.  He  is  identified  with  transcendental  reality and  is  said
  to  possess  the  power of reincarnation.  The  Buddha  is the
  Absolute Self running through all the so-called individual selves.  He
  is the  Noumenon  behind  all  phenomena.  .... 
Dry asceticism of the Hinayana is replaced by an enlightened and
  loving interest in this world. The nega­tive and individual conception
  of Nirvana is replaced by the positively blissful  and  universal 
  conception of it.  The  denial  of God  is  replaced with  the 
  Buddha’s  Divinity.  The  explosion  of  matter  and  mind  by
  reducing them to the series of momentary atoms and momentary ideas
  respectively, is replaced by the admission of the relative reality of
  both, the  transcendental  reality  being  the  Absolute,  the 
  Luminous  Body  of the  Buddha.  The greatness of the  Mahayana lies
  in its spirit of selfless service  of humanity,  its  accommodating
  spirit and its  missionary zeal.  The  Mahayanists  are  reasonably 
  proud  of their  faith  as  a  progressive and  dynamic  religion 
  which  throbs  with  vitality  because  it  has  the capacity  to 
  adapt  itself  with  the  changing  environmental  conditions,
  preserving its essentials  intact. .....
It (Hinayana schools) has ignored some important implications in the
  teachings of  the  Master and  has misinterpreted  many.  Some  of the
  main doctrines of the Buddha have been taken to their reductio ad
  absurdum pitch. As a matter of fact, there was nothing in the
  teachings of the Buddha which would seriously  militate against the 
  Upanisads. The Lamp of Dharma bequeathed by the Buddha to his 
  disciples was borrowed from the Upanisads. But the Hinayana made
  the constant and the luminous light of this Lamp flickering  and 
  faint.

This was refuted by Banaras Hindu University Professor T. R. V. Murti's statement (quoted below) in "Problem of Two Truths in Buddhism and Vedanta" (page 9):

It has been the fashion to consider that the differences between the
  Madhyamika śūnyatā and Brahman are rather superficial and even verbal,
  and that the two systems of philosophy are almost identical. At least
  Professor Radhakrishnan thinks so, and Stcherbatsky's and Dasgupta's
  views are not very different. I hold a contrary view altogether: that
  in spite of superficial similarities in form and terminology, the
  differences between them are deep and pervasive.

This was also refuted by eminent German indologist Professor Helmuth von Glasenapp's essay "Vedanta and Buddhism: A Comparative Study":

Nothing shows better the great distance that separates the Vedanta and
  the teachings of the Buddha, than the fact that the two principal
  concepts of Upanishadic wisdom, Atman and Brahman, do not appear
  anywhere in the Buddhist texts, with the clear and distinct meaning of
  a "primordial ground of the world, core of existence, ens realissimum
  (true substance)," or similarly.

Buddhism survived in the world through three routes - Tibetan Buddhism (and the preservation of the Tibetan Buddhist canon), East Asia (through transmission of Mahayana texts to China) and Theravada Buddhism's revival through Buddhism's own version of Adi Shankara, Buddhaghosa.
Buddhism has experienced a limited revival in India due to three reasons. The first is the conversion of low caste Dalits to Buddhism by Dr. Ambedkar (see this question). The second is the popularization of Vipassana meditation by S. N. Goenka. The third is the relocation of the Dalai Lama to Dharamsala, India.

user16: I think the invasions of India cannot be a sufficient reason for the decline of Buddhism. Much more is needed to destroy a religion than the destruction of holy sites (think about the followers!). Apart from that, Hinduism must have been at least as offensive to the piety of Muslim invaders - if not more offensive for obvious polytheistic idolatry - than Buddhism must have been, while Hinduism was not at all destroyed in India by the invaders. Moreover, the conquest of the south of the subcontinent could not happen before the fall of the religiously tolerant Vijayanagara Empire in the 17th century. I think the shortcomings of a monocausal theory of Muslim invaders destroying has been sufficiently shown.
For a time stretching from shortly after the Buddhas Nirvana to at the very least the beginning of the Common Era, Buddhism had been the most vigorous social, intellectual and religious movement of India (referring here to the Indian civilization, not to a state-entity). This flourishing opened out into a long decline paired with a "renaissance" of Hinduism (or rather: establishment of Hinduism) and Sanskrit learning paired with an increasingly powerful position of Brahmins in the political architecture of the Indian states. The reasons for this are rather complicated, you can get a not too much in-depth glance of them here: http://en.wikipedia.org/wiki/Decline_of_Buddhism_in_India
while if you want to know more in depth, check the sources given there or (maybe) any other academic source on history of Buddhism.

user254: India (the modern country) didn't exist when the Buddha was born.
The Buddha was born near modern-day Northern India or Nepal.
Wikipedia's Decline of Buddhism in the Indian subcontinent says,

[This] refers to a gradual process of dwindling and replacement of Buddhism in India, which ended around the 12th century. According to Lars Fogelin, this was "not a singular event, with a singular cause; it was a centuries-long process."
The decline of Buddhism has been attributed to various factors, especially the regionalisation of India after the end of the Gupta Empire (320–650 CE), which led to the loss of patronage and donations as Indian dynasties turned to the services of Hindu Brahmins. Another factor were invasions of north India by various groups such as Huns, Turco-mongols and Persians and subsequent destruction of Buddhist institutions such as Nalanda and religious persecutions. Religious competition with Hinduism and later Islam were also important factors.

This History of the Delhi Sultanates says,

Between 1160 and 1187, Muhammad al-Ghuri conquered the Ghaznavid Empire; he continued to expand his territory, and in 1192 he razed the Buddhist temple complex at Nalanda to the ground, in effect terminating the history of Buddhism in India.

It references a student's paper on the The History of Persecution of the Buddhist Faith, which includes,

Among the destroyed temples, 2/3 were Buddhists, and this was critical to Buddhism's ability to remain as a formal religion because they had already lost all but a few institutions in Nalanda, Odantapuri, and Vikramasila, over the previous centuries. (20)
The Nalanda University, a great Buddhist center of learning, was raided by Turkic Muslim invaders under Bakhtiyar Khalji, a general of the Turkish commander Qutb-ud-din Aybak, in 1193. He committed documented executions, harassed and tortured erudite monks, killing 15,000 scholars and 200 faculty of the University. (21) The campus and invaluable works of art including the images of the Buddha were destroyed and the enormous manuscript library of the University was burned down. He also destroyed the monastries in Vikramshila, which were in modern Bihar, as well as many monastries in Odantapuri in 1197. As he persecuted Buddhism, he supported Muslim missionaries and made the biggest number of converts to Islam under his reign. (22) By the end of the 12th century, many Buddhist monks retreated to Nepal, Sikkim, Tibet and Southern India. (23)
Persecution of Buddhism was accelerated in this period by Brahmin revivalists who kicked the Buddhist monks out of Buddhist monasteries and temples in order to transform the places into Hindu institutions; they were seeking protection from Muslim invasions by facilitating the installation of Brahmin gods. (24) No less than 1000 Buddhist temples were appropriated by Hindus, etc.

The (numbers) in the quote above are references to various history books etc.
There's a Wikipedia article (whose neutrality is currently disputed) titled Persecution of Buddhists which includes,

In 1200 Muhammad Khilji, one of Qutb-ud-Din's generals destroyed monasteries fortified by the Sena armies, such as the one at Vikramshila. Many monuments of ancient Indian civilization were destroyed by the invading armies, including Buddhist sanctuaries[45] near Benares. Buddhist monks who escaped the massacre fled to Nepal, Tibet and South India.[46]

... and continues in a similar way.
Perhaps these are the reasons: i.e. they're historical reasons.
It might seem as if it would be interesting to try to answer this question by discussing 'motive' (e.g. "why would people prefer to be Hindu than to be Buddhist"), comparative religious views (e.g. about 'just war'; and about how well the religion is integrated into the broader society at large), and comparative values (which religion is more useful, more attractive, more true) but I expect that would be subjective opinion, off-topic and unhelpful.

I'd also mention this answer to the question, Were Buddhists murdered en masse during the first Muslim conquest of India?
It references a thesis which says that Buddhism declined (not especially violently) during the 200 years or so following the first Muslim invasion -- i.e. in the 8th century AD -- mostly because urban merchants (who previously supported Buddhist monks) converted to Islam for various reasons; whereas the more-Hindu farmers were relatively unaffected.

802: <p>India is the original land of Buddhism, but it is followed/practiced more outside its country of birth. Are there any reasons for this?</p>
 985: <p>One of the main reasons for the decline of Buddhism in India is that it was engulfed by Hinduism in the years following the death of Buddha. </p>

<p>India, before the time of Buddha was a Brahminical society dominated by Brahmins and Kshatriyas who were at the top of a four level caste system, the others being Vaishyas followed by Shudras. The Indian society was rife with rites and rituals which could be only performed by Brahmins. A householder throughout his existence was supposed to carry out various sacrifices which included animal slaughter and thus the vasihyas and the shudras suffered the most during pre-Buddha period. Now Buddha's teachings gave no value to rites and rituals and also he preached non-violence. This made Buddhism much popular among the vaishyas and shudras. This was of course a threat to Brahminical dominance and thus the livelihood of the priestly class. Hence there were many attempts by the priestly class to sabotage Buddhist teachings even during the time of Buddha but they were unsuccessful. </p>

<p>After his death, they seem to have found success. Although Buddha negated the existence of God as is accepted in Hinduism, later Hindu fanatics made out Buddha himself to be a Hindu God. They declared him to be the 9th avatar of Vishnu, a Hindu God.</p>
 3602: <p>Brahmins were rarely involved in any wars (unless they were in the ruling position). The Vaishyas were never oppressed. In fact, they were wealthy land owners, money lenders etc. Hinduism is not a religion, its a culture. Hindus never oppressed any other religion, but have been oppressed. </p>

<p>Taken from <a href="http://www.esamskriti.com/essay-chapters/Why-did-Buddhism-vanish-from-India-1.aspx" rel="nofollow">http://www.esamskriti.com/essay-chapters/Why-did-Buddhism-vanish-from-India-1.aspx</a> :
I have listed down six reasons why Buddhism vanished from India. It is followed by the words of Swami Vivekananda &amp; Dr B R Ambedkar.</p>

<ol>
<li><p>The main cause was the neglect by the monks of this life and its values. While the Buddhist monks realized that everyone was not fit or could not become a monk or nun, they paid attention only to the life of a monk and not to the life of a householder. Which meant that they focused on the life of a monk, which is a life of inwardness as compared to that of a householder, which is one of outwardness. Now, both these aspects need examination, study, guidance and control. It is not enough to tell a householder that this existing life is only a stepping-stone to the life of a monk. Why and how is it so and what relation it bears to realities has to be explained. Instead Buddhist philosophers began to teach that this life was nothing but a value of tears and misery. While some forms of Vedanta taught the same philosophy, the attitude of Mimamsa (philosophy of action) and the Epics saved Hinduism from the fate that overtook Buddhism in India. Many great Indians were impacted by spiritual teachings but “unless there were some codes extolling the values of the world, they tended to become one-sidedly inwardly”.</p></li>
<li><p>Another reason was the admission of women into monasteries and the more or less indiscriminate conversion of men, women into monks and nuns. While true renunciation and celibacy were appreciated, people wanted to see them well practiced. When people supported these monasteries with their hard-earned money, they did not want its residents to live in luxury and enjoyment, virtues, which were condemned. If monks and nuns had lived by the rules that they were taught, people would have supported them inspite of any hardship that they had to face.</p></li>
<li><p>The next reason was the deterioration in the political and economic life of the country. Monasteries were supported by the people and the Kings e.g. Ashoka. Now, when a dynasty fell or a king died, the next in line might not give the same degree of support. The king’s thinkers realized that their defeat was due to the loss of their best fighters, leaders, who had become monks. This made the country an easy prey to the foreign invader. Coincidence or otherwise, India’s first foreign invasion by the Greeks took place in 327 B.C. a couple of centuries after Emperor Asoka’s peace movement.</p></li>
<li><p>Buddhism existed in the monasteries and unlike the dharmaasutras (ethical codes) lacked a moral code. So when monasteries disappeared, Buddhism disappeared. The invasion of the Muslims and the ruthless destruction of Buddhist monasteries extinguished the lamp of Buddhism in North India. The wanton destruction of the great monastery of Uddandapura (Bihar) and the wholesale massacre of its monks might make us visualize how the great monasteries of Nalanda, Vikramasila and others met with a tragic end.</p></li>
<li><p>The extreme asceticism practiced and popularized by both Buddhism and Jainism disturbed the social life of India. Magadha, the seat of many imperial dynasties, became Bihar, the land of monasteries (viharas). There was nothing in these religions to emphasize the importance of life in this world and its values. These causes led to a bloodless revolt by the orthodox in the eight-century a.d. The revolt was staged from two sides, the Brahmanic and the Upanisadic. Kumarila was the leader of the former and Sankara of the latter. Kumarila succeeded in reviving a strong positive attitude towards the world and its values and all that could be called human and activistic. On the other hand, Sankara said that everything that was good in Buddhism already existed in the Upanishads. In fact, Gaudapada, the grand teacher of Sankara, unified the current spanda (vibration) doctrine of Saivism, the vijnana (mind) doctrine of the Buddhists and the Atman doctrine of the Upanishads in his Mandukyakarikas and made the way easy for Sankara to assimilate and absorb Buddhism. Thus, there remained no justification for its separate existence in India; it had no social ethics and consequently, no hold over society. It could not stand alone as a spiritual discipline as it was shown to be part of the Upanishads.</p></li>
<li><p>Quoting Swami Vivekananda “ Thus, inspite of preaching mercy to animals, inspite of the sublime ethical religion, inspite of the discussions about the existence or non-existence of a permanent soul, the whole building of Buddhism tumbled down piece-meal and the ruin was simply hideous. The most hideous ceremonies, the most obscene books that human hands ever wrote or the human brain ever conceived, have all been the creation of the degraded Buddhism. The Tartars and the Baluchis and all the hideous races of mankind that came to India, became Buddhists and assimilated with us, brought their national customs and the whole of our national life became a huge page of the most horrible, bestial customs. Sankara came and showed that the real essence of Buddhism and that of Vedanta are not very different but that the disciples did not understand the master and have degraded themselves, denied the existence of soul and one God and have become atheists. That was what Sankara showed and all the Buddhists began to come back to their old religion”.</p></li>
<li><p>Buddhism adopted various thoughts and beliefs between the first century B.C. and the sixth century a.d. Some Buddhists adopted the tantric sadhanas and distorted them for the sake of enjoyment and comfort. The highly advanced philosophy of tantric sadhana is difficult to understand without the guidance of a proper teacher. This undigested knowledge of tantra, including the use of wine, meat, fish, gestures and physical union led these Buddhist followers to their downfall. Also, the distortions of Buddhism produced a variety of schools, which were not pure Buddhist schools but contained a variety of practices. To give you an idea of the syntheses between Vedanta and Buddhism, the concept of Maya in Vedanta in borrowed from Buddhism. Sankara accepted the logical connotation of Maya just as it was given by the Buddhists. Jainism was saved by tacitly allowing its members to become part of the Hindu fold by adopting rules of conduct of the third caste, namely Vaisyas or traders.</p></li>
<li><p>Quoted from ‘Dr Ambedkar Life &amp; Mission by Dhananjay Keer’. Dr B R Ambedkar addressed delegates of Young Men’s Buddhist Association in May 1950 at Colombo on ‘Rise &amp; fall of Buddhism in India’ - ‘Buddhism in its material force had disappeared. But as a spiritual force it still exists’. As regards Hinduism he said it went through three phases, Vedic religion, Brahmanism and Hinduism. It was during the Brahmanism period that Buddhism was born. It was not true that after the days of Shankaracharya Buddhism was dead in India. It was going on for years together. In fact Shankaracharya and his teacher were both Buddhists he added. While he was digging material on the subject for the decline/vanish of Buddhism from India the reasons were – adoption of some rituals &amp; practices from Buddhism by the Vaishnava &amp; Shaiva cults, which were vociferous in their propaganda against Buddhism. During the invasion by Allauddin Khilji thousands of priests in Bihar were massacred and consequently some of them fled for their lives to Tibet, China &amp; Nepal. In the meanwhile, the majority of Buddhists went over to Hinduism. The third cause was that Buddhism was difficult to practice while Hinduism was not. Reason four was that the political atmosphere in India had been unfavorable to the advancement of Buddhism he concluded.</p></li>
</ol>

<p>But according to Hindu scholars the fall of Buddhism was due to many reasons. Owing to universalistic ambition its spread was everywhere but it had geographical center nowhere. It discarded all national gods &amp; godmen &amp; proclaimed Buddha the greatest of all gods. As long as it reacted as a reformative flank in India, Buddhism gained ground but when it began to act against the Vedic religion, which was the national religion of the majority, Buddhism lost sympathy in India. The Vedic Hindus fought the Muslims bravely and did not flee to any other country. But the Buddhists when attacked, having a center nowhere, fled to different countries and even it is said acclaimed the invasion of India by non-Hindus with the ringing of bells. Besides its godlessness, its over-emphasis on redemption, its sad tone, its unconcern with the world &amp; neglect of family checked rather than fostered enterprise. Quote ends. </p>

<p>Books referred to 1. Introduction to Comparative Philosophy by P T Raju. 2. History &amp; Culture of Indian People by Bhartiya Vidya Bhavan.</p>
 471: <p>Buddhism declined from India in the past, due to assimilation.</p>

<p>Firstly, Buddhism had flourished in ancient India, Pakistan and Afghanistan, but eventually, the Buddhist locals became subject to "hard" assimilation by foreign invaders. See the Wikipedia articles <a href="https://en.wikipedia.org/wiki/Buddhism_in_Afghanistan" rel="nofollow noreferrer">Buddhism in Afghanistan</a>, <a href="https://en.wikipedia.org/wiki/Buddhism_in_Pakistan" rel="nofollow noreferrer">Buddhism in Pakistan</a> and also <a href="https://en.wikipedia.org/wiki/Decline_of_Buddhism_in_the_Indian_subcontinent#Decline_under_Islamic_rule" rel="nofollow noreferrer">this article section</a>. This is not unlike the disappearance of Zoroastrianism from its homeland of Persia.</p>

<p>Secondly, Buddhism suffered from "soft" assimilation into Hinduism.</p>

<p>Before Buddhism (around 500 BCE), the historic Vedic religion was mainly focused on sacrifices and rituals. It did not have robust philosophy and metaphysics. After encountering Buddhism and the shramanic movement (which includes Jainism), the Upanishadic movement rose in India from 500 - 200 BCE. After this, the Vedanta movement rose as part of Classical Hinduism from 200 BCE - 1100 CE. The Hindu text Brahma Sutras that attempted to refute Buddhist and other non-Hindu teachings, was written between 200 BCE and 400 CE. See <a href="https://buddhism.stackexchange.com/q/7426/471">this question</a> for details.</p>

<p>Adi Shankara, the famous reformer who revived Hinduism and consolidated Advaita Vedanta, lived around 800 CE. Advaita is the philosophical school of Vedanta that is closest to Buddhism. Even in those days, Hindu philosophical opponents of Shankara accused of him of being a crypto-Buddhist. Through Advaita Vedanta, Hinduism assimilated Buddhism by adapting parts of its philosophy unto itself. Some Hindus even claim that Mahayana's emptiness (shunyata) is exactly the same as Advaita's Transcendental Ultimate Reality. See <a href="https://buddhism.stackexchange.com/a/28335/471">this answer</a> for details.</p>

<p>On the other hand, the Dvaita Vedanta school, which has completely incompatible philosophy to Buddhism, assimilated Buddhism by making Buddha an incarnation of the Hindu deity Vishnu. The Hindu text Bhagavata Purana written between 500 CE and 1000 CE states that Vishnu incarnated as Buddha to delude the immoral atheists into living ethical lives. According to them, the Buddha tricked atheists into taking refuge in the Buddha and serving the Buddha, who is actually God in disguise. More info on this is available on <a href="http://www.harekrsna.com/philosophy/gss/sadhu/religions/buddhism.htm" rel="nofollow noreferrer">this page</a>.</p>

<p>Hindu philosophers also claimed that  Brahma Sutras, Bhagavata Purana and other such texts were divinely authored a long time before the arrival of the Buddha. This was one way to convince Buddhists that Vishnu's incarnation as the Buddha was foretold by the Hindu scriptures even before his birth. This was debunked by professional academicians.</p>

<p>In my personal opinion (which may lean towards Theravada), the final reason for Buddhism's "soft" assimilation into Hinduism is the development of concepts in Mahayana Buddhism that sound very similar to concepts in Hinduism, like the Eternal Buddha, Adi Buddha (Primordial Buddha), Buddha Nature (tathagatagarbha), storehouse consciousness (alayavijnana) and beings like Avalokiteshvara etc.</p>

<p>In his book "<a href="https://archive.org/details/IndianPhilosophyACriticalSurvey" rel="nofollow noreferrer">Indian Philosophy: A Critical Survey</a>" on pages 63 - 64, Professor Chandradhar Sharma wrote very misleadingly about Buddhism:</p>

<blockquote>
  <p><strong>Buddha is here transformed (by Mahayana Buddhism) into God and  worshipped  as 
  such.  He  is  identified  with  transcendental  reality and  is  said
  to  possess  the  power of reincarnation.  The  Buddha  is the
  Absolute Self running through all the so-called individual selves.  He
  is the  Noumenon  behind  all  phenomena.</strong>  .... </p>
  
  <p>Dry asceticism of the Hinayana is replaced by an enlightened and
  loving interest in this world. The nega­tive and individual conception
  of Nirvana is replaced by the positively blissful  and  universal 
  conception of it.  <strong>The  denial  of God  is  replaced with  the 
  Buddha’s  Divinity.</strong>  The  explosion  of  matter  and  mind  by
  reducing them to the series of momentary atoms and momentary ideas
  respectively, is replaced by the admission of the relative reality of
  both, <strong>the  transcendental  reality  being  the  Absolute,  the 
  Luminous  Body  of the  Buddha</strong>.  The greatness of the  Mahayana lies
  in its spirit of selfless service  of humanity,  its  accommodating
  spirit and its  missionary zeal.  The  Mahayanists  are  reasonably 
  proud  of their  faith  as  a  progressive and  dynamic  religion 
  which  throbs  with  vitality  because  it  has  the capacity  to 
  adapt  itself  with  the  changing  environmental  conditions,
  preserving its essentials  intact. .....</p>
  
  <p>It (Hinayana schools) has ignored some important implications in the
  teachings of  the  Master and  has misinterpreted  many.  Some  of the
  main doctrines of the Buddha have been taken to their <em>reductio ad
  absurdum</em> pitch. <strong>As a matter of fact, there was nothing in the
  teachings of the Buddha which would seriously  militate against the 
  Upanisads. The Lamp of Dharma bequeathed by the Buddha to his 
  disciples was borrowed from the Upanisads.</strong> But the Hinayana made
  the constant and the luminous light of this Lamp flickering  and 
  faint.</p>
</blockquote>

<p>This was refuted by Banaras Hindu University Professor T. R. V. Murti's statement (quoted below) in "<a href="https://archive.org/details/ProblemOfTwoTruthsInBuddhismAndVedantaMrvynSprungArticles" rel="nofollow noreferrer">Problem of Two Truths in Buddhism and Vedanta</a>" (page 9):</p>

<blockquote>
  <p>It has been the fashion to consider that the differences between the
  Madhyamika śūnyatā and Brahman are rather superficial and even verbal,
  and that the two systems of philosophy are almost identical. At least
  Professor Radhakrishnan thinks so, and Stcherbatsky's and Dasgupta's
  views are not very different. I hold a contrary view altogether: that
  in spite of superficial similarities in form and terminology, the
  differences between them are deep and pervasive.</p>
</blockquote>

<p>This was also refuted by eminent German indologist Professor Helmuth von Glasenapp's essay "<a href="https://www.accesstoinsight.org/lib/authors/vonglasenapp/wheel002.html" rel="nofollow noreferrer">Vedanta and Buddhism: A Comparative Study</a>":</p>

<blockquote>
  <p>Nothing shows better the great distance that separates the Vedanta and
  the teachings of the Buddha, than the fact that the two principal
  concepts of Upanishadic wisdom, Atman and Brahman, do not appear
  anywhere in the Buddhist texts, with the clear and distinct meaning of
  a "primordial ground of the world, core of existence, <em>ens realissimum</em>
  (true substance)," or similarly.</p>
</blockquote>

<p>Buddhism survived in the world through three routes - Tibetan Buddhism (and the preservation of the Tibetan Buddhist canon), East Asia (through transmission of Mahayana texts to China) and Theravada Buddhism's revival through Buddhism's own version of Adi Shankara, <a href="https://en.wikipedia.org/wiki/Buddhaghosa" rel="nofollow noreferrer">Buddhaghosa</a>.</p>

<p>Buddhism has experienced a limited revival in India due to three reasons. The first is the conversion of low caste Dalits to Buddhism by Dr. Ambedkar (see <a href="https://buddhism.stackexchange.com/q/14044/471">this question</a>). The second is the popularization of Vipassana meditation by S. N. Goenka. The third is the relocation of the Dalai Lama to Dharamsala, India.</p>
 16: <p>I think the invasions of India cannot be a sufficient reason for the decline of Buddhism. Much more is needed to destroy a religion than the destruction of holy sites (think about the followers!). Apart from that, Hinduism must have been at least as offensive to the piety of Muslim invaders - if not more offensive for obvious polytheistic idolatry - than Buddhism must have been, while Hinduism was not at all destroyed in India by the invaders. Moreover, the conquest of the south of the subcontinent could not happen before the fall of the religiously tolerant Vijayanagara Empire in the 17th century. I think the shortcomings of a monocausal theory of Muslim invaders destroying has been sufficiently shown.</p>

<p>For a time stretching from shortly after the Buddhas Nirvana to at the very least the beginning of the Common Era, Buddhism had been the most vigorous social, intellectual and religious movement of India (referring here to the Indian civilization, not to a state-entity). This flourishing opened out into a long decline paired with a "renaissance" of Hinduism (or rather: establishment of Hinduism) and Sanskrit learning paired with an increasingly powerful position of Brahmins in the political architecture of the Indian states. The reasons for this are rather complicated, you can get a not too much in-depth glance of them here: <a href="http://en.wikipedia.org/wiki/Decline_of_Buddhism_in_India">http://en.wikipedia.org/wiki/Decline_of_Buddhism_in_India</a>
while if you want to know more in depth, check the sources given there or (maybe) any other academic source on history of Buddhism.</p>
 254: <p>India (the modern country) didn't exist when the Buddha was born.</p>
<p>The Buddha was born near modern-day <a href="http://en.wikipedia.org/wiki/Gautama_Buddha#Conception_and_birth" rel="noreferrer">Northern India or Nepal</a>.</p>
<p>Wikipedia's <a href="https://en.wikipedia.org/wiki/Decline_of_Buddhism_in_the_Indian_subcontinent" rel="noreferrer">Decline of Buddhism in the Indian subcontinent</a> says,</p>
<blockquote>
<p>[This] refers to a gradual process of dwindling and replacement of Buddhism in India, which ended around the 12th century. According to Lars Fogelin, this was &quot;not a singular event, with a singular cause; it was a centuries-long process.&quot;</p>
<p>The decline of Buddhism has been attributed to various factors, especially the regionalisation of India after the end of the Gupta Empire (320–650 CE), which led to the loss of patronage and donations as Indian dynasties turned to the services of Hindu Brahmins. Another factor were invasions of north India by various groups such as Huns, Turco-mongols and Persians and subsequent destruction of Buddhist institutions such as Nalanda and religious persecutions. Religious competition with Hinduism and later Islam were also important factors.</p>
</blockquote>
<p>This <a href="http://www.zum.de/whkmla/region/india/xdelhisult.html" rel="noreferrer">History of the Delhi Sultanates</a> says,</p>
<blockquote>
<p>Between 1160 and 1187, Muhammad al-Ghuri conquered the Ghaznavid Empire; he continued to expand his territory, and in 1192 he razed the Buddhist temple complex at Nalanda to the ground, in effect terminating the history of Buddhism in India.</p>
</blockquote>
<p>It references a student's paper on the The History of Persecution of the Buddhist Faith, <a href="http://www.zum.de/whkmla/sp/1112/sbk/sbk2.html#iii14" rel="noreferrer">which includes</a>,</p>
<blockquote>
<p>Among the destroyed temples, 2/3 were Buddhists, and this was critical to Buddhism's ability to remain as a formal religion because they had already lost all but a few institutions in Nalanda, Odantapuri, and Vikramasila, over the previous centuries. (20)</p>
<p>The Nalanda University, a great Buddhist center of learning, was raided by Turkic Muslim invaders under Bakhtiyar Khalji, a general of the Turkish commander Qutb-ud-din Aybak, in 1193. He committed documented executions, harassed and tortured erudite monks, killing 15,000 scholars and 200 faculty of the University. (21) The campus and invaluable works of art including the images of the Buddha were destroyed and the enormous manuscript library of the University was burned down. He also destroyed the monastries in Vikramshila, which were in modern Bihar, as well as many monastries in Odantapuri in 1197. As he persecuted Buddhism, he supported Muslim missionaries and made the biggest number of converts to Islam under his reign. (22) By the end of the 12th century, many Buddhist monks retreated to Nepal, Sikkim, Tibet and Southern India. (23)</p>
<p>Persecution of Buddhism was accelerated in this period by Brahmin revivalists who kicked the Buddhist monks out of Buddhist monasteries and temples in order to transform the places into Hindu institutions; they were seeking protection from Muslim invasions by facilitating the installation of Brahmin gods. (24) No less than 1000 Buddhist temples were appropriated by Hindus, etc.</p>
</blockquote>
<p>The (numbers) in the quote above are references to various history books etc.</p>
<p>There's a Wikipedia article (whose neutrality is currently disputed) titled <a href="http://en.wikipedia.org/wiki/Persecution_of_Buddhists" rel="noreferrer">Persecution of Buddhists</a> which includes,</p>
<blockquote>
<p>In 1200 Muhammad Khilji, one of Qutb-ud-Din's generals destroyed monasteries fortified by the Sena armies, such as the one at Vikramshila. Many monuments of ancient Indian civilization were destroyed by the invading armies, including Buddhist sanctuaries[45] near Benares. Buddhist monks who escaped the massacre fled to Nepal, Tibet and South India.[46]</p>
</blockquote>
<p>... and continues in a similar way.</p>
<p>Perhaps these are the reasons: i.e. they're historical reasons.</p>
<p>It might seem as if it would be interesting to try to answer this question by discussing 'motive' (e.g. &quot;why would people prefer to be Hindu than to be Buddhist&quot;), comparative religious views (e.g. about 'just war'; and about how well the religion is integrated into the broader society at large), and comparative values (which religion is more useful, more attractive, more true) but I expect that would be subjective opinion, off-topic and unhelpful.</p>
<hr />
<p>I'd also mention <a href="https://skeptics.stackexchange.com/a/37575/2703">this answer</a> to the question, <em>Were Buddhists murdered en masse during the first Muslim conquest of India?</em></p>
<p>It references a thesis which says that Buddhism declined (not especially violently) during the 200 years or so following the first Muslim invasion -- i.e. in the 8th century AD -- mostly because urban merchants (who previously supported Buddhist monks) converted to Islam for various reasons; whereas the more-Hindu farmers were relatively unaffected.</p>
