One factor is the number of selections from Kota. One of my friends told me that the coaching classes there paint a rosy picture of IITB and IITD at the expense of IITK and IITKGP. This time however, even Delhi has witnessed decline. May be that has something to do with placement performance of IITD in last two years.
Other factor of course is location. Candidates prefer cities like Mumbai and New Delhi to Kanpur. IITM takes a beating here because it is in South India. Most of selections from South India are from Andhra and students from that state are in considerable number even in IITK, so I suppose the number would be even more in IITB.
The third factor is the placement record. Clearly the placements are best in IITB.
That is what I could think of right now as reasons for this skewed distribution. However, JEE chairman of IITB offered the reason to be introduction of "minors". Minors were already in existence in Kgp and IITM and still Kgp could not get a single student in top 100. In my opinion this trend is very bad for other IITs. Each IIT has its own fort. Like CS department of IITK is best and same is the perception about Chemical of IITB. Civil is best in Roorkee. So anyone must consider his interest before deciding on any IIT and not the craze which these coaching wallahs create without having any experience of either industry or research.
P.S. : These are my own views expressed on the basis of perception and information from some of my fellow students. If you feel these are not correct feel free to drop a comment.
