I believe this is to counter the tendency of students to dress shabbily in shalwar kameez.
Buttons opened, unironed, dirty, with the payenchay going way under the feet, and open shoes, this was common sight during my undergrad days even. I used to wear shalwar kameez myself but with closed shoes, ironed it, wear it in a proper manner and some pride at least.
At least this is the only sense I make.
Now the decision maker just thought we'll outright ban it rather than making people adhere to the proper manner.
Now don't mistake me, I'm not saying impose restrictions as if it's an army mess (MGM, Artillery mess, although I think they have relaxed on the rules too), but still be tidy.