Because Large or larger makes life a lot more complicated for game designers. It's easier when everyone has the same size, and thus the same reach and area of effect for certain kinds of things.
Note, however, that this pattern is not totally inviolate, based on the recent UA. Barbarians with the Path of the Giant can rage to become Large or even Huge at high level. But this is an opt-in thing, rather than an always-on thing, and that makes a big difference. Or, if you prefer, this means being Large size is so good, having only a small number of rounds you can do that each day is a class feature all by itself.