Because they have far fewer places than they do suitable applicants. *Shrugs* It happens, plenty of people get into top universities but rejected from others, I know somebody who was ONLY accepted into Cambridge.
It's nothing against you or your achievements, they just don't have the places. Perhaps they had an issue with a tiny part of your personal statement. Eventually they have to weed people out for the smallest of things.
Don't worry about it, move on, be happy with your other offers.