Bulbasaur chose to not evolve. 3nodding The others he never really used also chose to leave.
Pidgeot wanted to protect Pidgeotto and Pidgey from Spearow and Fearow. Primape wanted to train for P1 Grand Prixes. Butterfree found a mate. Charizard comes back upon occasion, he's just training at the Charizific Valley because it gives him more experience than traveling with Ash. Lapras was always meant to go away. The only reason why Lapras traveled with Ash and friends was to find its family. Muk and Snorlax should be at Prof. Oak's lab like most of Ash's other Pokémon.
This is all I know so far since Pokémon left KidsWB.
