Kinda like how a human kid turned into a Kadabra, but not an Abra or Alakazam. I don't get why the Pokédex singles out specific Pokémon like that.

[Maybe they felt Blastoise's cannons were more interesting. Or maybe there's some insane Pokéverse logic going on. Heck if he knows.]

And don't butterflies only live a few months? Maybe you're onto something.