At first, I would like you to know that there are certain causes behind the decline in quality. I've mentioned below a few of them here. 

• Before there was no reservation at BIT Mesra. After 2006 they started a reservation policy. It is the first and foremost reason that became a harbinger for the others. 
• According to many student reviews, the faculty at the institute are not that good. There is always one professor out of 20 who can be designated as a professor in every department. 
• If you are in the T&P department then your placement is guaranteed along with the placement of your friends. As you will be in the head position and have easy access to the placement exams. 

Being said that, the institute has many pros too.