1) it's still fresh because roamer and demo fast rollouts are cool, every class is mobile, you need a medic, you have a flank, and you need a combo. I can't think of any class that can permanently replace the composition already there without impacting the game negatively. many maps such as sunshine process etc are designed with normal 6s in mind. uif you ran different class composition you would take away the midfight meta, can you imagine one team showing up to mid super early and the other coming after mid is half capped because pyro spy and sniper are just slower classes? also you're never going to be able to expect what the other team is running, and that would bascally make scrimming pointless since fuck knows who's going to run what. and even then the meta will develop again and people will realize that the current meta works best, plus prolander was already tried and failed.
it's a stupid comparison but in really old cs people had limits on how many guns you could run like you weren't allowed to run 4xAK or whatever and then valve made MM without the restrictions and there's a new meta and it's "stale" again. it's not a perfect analogy but a new meta will evolve and people like you will say it's stale again. i'm not really sure if this makes sense as a comparison, though.
2) there's nothing stopping people from running whatever classes they want in MM, I mean it's already goofy enough that adding class limits might be ok but people are going to get mad that they can't play whatever class that they want and abandon which causes the whole lobby to end.