Pokemon In Super Smash Bros: How They're Selected
Given there’s over 600 Pokemon, it isn’t feasible to include everyone single one in Super Smash Bros, for obvious reasons.
So how are Pokemon selected for inclusion in the series?
"Well first of all, we talk with the Pokemon company," explained designer Masahiro Sakurai when we put the question to him. "What’s the hot Pokemon? What Pokemon are in the movies right now? And really do a lot of research on that front.
via Pokemon In Super Smash Bros: How They're Selected - NowGamer.