Firstly, let me make it clear that Kanpur, Delhi, and Mumbai are preferred over IIT Kharagpur and IIT Madras for almost every branch. The main reasons are as follows.
Placements: The placements at IIT Kanpur are slightly better than Kharagpur.
Trend: While choosing an institute, students tend to choose an institute based on the past year’s trend. Since the campus placement record for the top 5 IITs remains the same over the past 5 years, students tend to go to these institutes.
These are a few of the reasons why students prefer IIT Kanpur for CS over IIT KGP and IIT Madras.