Why could an accelerated education be seen as negative? I've had discussions with professors in conjunction with my postgraduate application. Many were very skeptical at the fact that I completed one BSc and two MSc within 4 years. This would normally take at least 5-6 years in my country. I explained that I did it because I was very interested in my subjects. However, they thought it was strange and concerning since I had a small portion of "bad grades". This can be explained by a combination of heavy workload, bad family situation, and poor mental health during one or two semesters. I did not mention the latter parts due to social stigma. As a result, professors said I may have "learned more", i.e. gotten straight A's, if I'd have studied a year or two longer like most people. Other than this I have a very good record, which made this seem strange to them.

I live in a small country that has a lot of ex-Soviet professors, many of which believe that a student must have straight A's and a standard educational history to succeed. However, in my country universities are very diverse in terms of expectations and grading. Every university has their own grading scale and level of education. My university, for instance, likes to fail 50% every module to weed out bad students. Sometimes, up to 85% fail, because they give undergraduate students graduate modules because "life's tough, just learn it".

So, depending on where you study, your grades will be wildly different, and it can be hard to convert between places. There's even massive variation within my department alone, where they have the same module but different formats depending on who takes it. This means that an architect taking linear algebra who gets an A may be counted as a C for someone who studies physics, for instance.

Given this knowledge of the educational system, what could be the reason that professors dislike accelerated education? If asked at my university, where it is common, they'd just say good job. When asked at the universities I applied to, it's almost unheard of. How can one highlight the advantages of having taken a fast educational route?
Given this knowledge of the educational system, what could be the reason that professors dislike accelerated education? 


There is not a good reason, and most professors do not hold that view.


  How can one highlight the advantages of having taken a fast educational route?


I have not have much success with that, and I am not sure you should try.

The advantage of accelerated education goes to you personally, not your supervisor.  If you complete you degree at a younger age, then you work for more years between that degree and your retirement.  As a result, you achieve more and earn more.  Graduate school supervisors will not care much.  It's true that students who have successfully accelerated their education are hard workers, but they are not the only hard workers.