Ash Ketchum starts his journey to become a Pokemon master with three Pokemon that go on to become some of his best friends – Pikachu, Bulbasaur and Squirtle. None of the three ever evolve into their Stage 1 forms. There can be several reasons for this.
Firstly, Ash’s Pokemon, in addition to Pikachu, are the face of the entire franchise. Bulbasaur, Squirtle and even Charmander are some of the cutest and most-loved characters from the series. Their evolved forms are not as cute as their base forms and do not attract quite the same adulation from fans, who are mostly children.
A second reason for Ash’s Pokemon not evolving goes with the central theme of the show – gathering strength from within and defeating the next big challenge.
Pikachu refused evolution with the help of the thunder stone and defeated Raichu by himself and Bulbasaur didn’t evolve as he wanted to continue his journey with Ash and his fellow Pokemon, instead of becoming a mythical protector of the forest by evolving into Venusaur. Even Squirtle doesn’t evolve due to similar emotional reasons and his attachment to Ash.