>>15331274To be honest it all depends on how easy it is to animate the starter (since they will appear regularly), and their general popularity (cuteness-bias). (Japan-wise) Sometimes story canon comes into this as well.
Squirtle has canon issues, it cannot evolve due to its association with the Squirtle Squad.
Totodile is the cutest of its line, it also doesn't take battling seriously and is a joker. Comic relief, doesn't evolve.
Ash did not own a Mudkip. Brock's Mudkip evolved into Marshtomp and slowly got less focus as the show went on. Mudkip is popular than Marshtomp, but since it wasn't under Ash's team, this wasn't that much of an issue to deal with.
Dawn's Piplup did not evolve because in addition to being comic relief, it's also the cutest of its line.
It's easier to animate bi-pedal starters than quadruples, to start off. This is why Oshawott did not evolve into Samurott, and why Samurott rarely appeared in the show itself. Oshawott could have evolved into Dewott, but it simply isn't as popular as Oshawott.
Meanwhile, Froakie actually has a backstory similar to Ash's Fire starters, so the potential for it to evolve is high. It's "cute", but in a different essence of cute compared to say, Piplup or Oshawott.
