Its due to exactly 1 issue.
Us as society tieing one’s success to them being from IIT/NIT.
People have a tough time digesting the fact that a person who is not from IIT/NIT is doing well in life. And this is actually the case with VIT. Students from VIT are very well equipped to do well in life. For instance, from my batch all of my friends who were sincere are working in reputed organizations, some went to Army, a lot of them went to USA and are doing extremely well. Heck, I am doing amazing.
I think this is the only reason why VIT has a bad reputation. Its a fantastic institute, I will be ever grateful to the institute.