Right. I understand that, but say they did that with Diablo II. And one of the quests was for necros to go kill so major badass guy. You'de be stuck with only necros, not to mention you'de most likely want to leave the game and join a new one, or create a new one specifically for just that. Then of course you have to deal with the assholes that join the game anyway.
In theory, it could work, if they added a class filter to create game, but they most likely won't.