Two hypotheses readily come to one's mind. One, PSUs concentrate their attention where they can catch their victims more easily. Therefore, their focussed targets are those who have been "gullible twice over".
The second and the more persuasive hypothesis is that the higher average age of IIT-IIM combos makes them cognitively more receptive to public sector culture. Theoretical support for this hypothesis is seen in Prof. Timothy Salthouse's recent discovery that old age starts at 22. (Actually, the Professor says that cognitive capacity is at its best at 22 years of age, plateaus for some time and there is a noticeable decline from 27). Average age of an IITian is 21 at graduation. Average age of a non-engineer at graduation from IIM is also 21. Typical age of an IIT cum IIM graduate is 23 when he / she passes out of IIM. At 23, if Timothy Salthouse is right, a person's cognitive ability is likely to have started declining. Does it make public sector a better place to work in for such alumni?
Timothy Salthouse is unlikely to be proved wrong anytime soon because any challenge from persons on the wrong side of 22 is likely to be dismissed as a sinister response from the cognitively - challenged.