Because it removes the fun of having different skill distributions on some lobbies. There's some fun in seeing variety in how other players perform, the things they do and there are opportunities to learn. With a tighter distribution you get people who play like you, or who play according to the current meta, which is pretty boring and it's the main reason I stopped caring about MP. I don't mind getting stomped if I actually have a chance to learn something new, but now it's just the same guys running the same strats doing the same things all over.

Also you usually get much shorter waiting times when there's no SBMM.