Only 21 Pokemon on the roster? Even Pokemon stadium for N64 had all 150(I never tried to import a Mew). Get your act together.
Except this is a fighting game closer to Tekken than classic Pokémon.
For each character they need a move list, which includes the input needed to trigger it, its appearance, effect (Does it grant immunity? Does it break defense? Does it launch projectiles? Can the attack be cancelled?), and timing (When does the effect trigger? When does the attack deal damage? When can the character move again?), and much more.
It's a lot of work for a single character. Not to mention that they all need to be balanced.
It's not like Pokémon Stadium, where you only need each Pokémon to have a single attack animation, and each attack to have only a single visual effect.