Based on what I'm reading here I'm clearly wrong, but I thought the reason for not including the whole roster was that every pokemon needed to get an entire new model and animations and stuff for this game? That seems like it makes far more sense to me, at least. The last two generations basically ran on the same hardware so models could just be moved over mostly as-is, but for this one they'd have to make an enormous amount of new models if they included all the pokemon.