As technology advances it allows smartphone manufacturers to create bigger phones. Some manufactures already wanted to create bigger smartphones but were waiting for the proper technology.
Smartphones are replacing personal computers
Many people buy smartphones instead of buying a personal computer. Those people expect the phone to be big enough to allow them to do most of the things they used to do on a PC.
Customers want the benefits of a Tablet
Samsung said that many customers wanted the benefits of a tablet without buying a tablet.
Customers want bigger phones
Smartphone manufactures have realized that customers want larger phones.
Larger models succeeded
As many large phones became successful other manufactures started copying them and creating large phones as well. (See Why smartphones are so popular?)
Phones are becoming more visual
Phones are shifting away from audio based functions to display based functions. This kind of transition made people realize that they want a larger phone with a larger display.
Advancement in battery technology
Early batteries couldn’t power a phone with a big screen easily. As technology advanced manufactures were allowed to create phones with bigger screens. (See Why do people buy expensive phones?)