i think if jessie and james teamrocket transitioned theyd just switch their names and call it a day. or this already happened. this is my one true belief
Doylist explanation: They keep making new electric mouse pokemon because they're frantically trying to recreate the success of Pikachu. Or at least because it's tradition at this point.
Watsonian explanation: A major predator of rodents is birds of prey. A mouse that can deliver electric shocks to flying-types has a much higher chance of living to produce more offspring. Thus, convergent evolution fills this niche in every region. "Pikachu" is just the pokemon universe equivalent of "crab".