Pikachu doesn’t evolve into Raichu nor does Snorlax/Mr Mime evolve from their pre-vos

That's different. Pikachu is the game's mascot, and Snorlax and Mr. Mime's pre evolutions are baby pokemon.

Psyduck is simply more iconic than its evolution(which, unite seems to be leaning towards popular pokemon atm). or maybe it’s a nod to Misty’s voice actor?

We need yvletal