The big excuse they've always trotted out is that Overwatch's custom-made engine (as opposed to the extremely flexible Source engine) is basically capable of doing nothing but 6v6 hero fights and anything else is very hard to program from scratch.
Oh ok, I guess I've heard that before. I guess it kind of makes sense in light of OW being the cobbled together remnants of a failed MMO. Hopefully OW2 or whatever it is allows them to bring more flexibility into the engine (and for my own personal benefit, doesn't increase the system requirements too much).