>>39292417It was never stated why Bulbasaur didn't evolve.
The most correct explanation is that the writers didn't make evolve because he was more popular than Ivysaur and Venusaur.
They made an episode of Ash's Pikachu refusing to evolve for the same reason, the writers just wanted to tell the viewers that these two will never evolve.
Bulbasaur himself was way too popular in Japan that the studio kept him on screen with Ash the longest after Pikachu and he kept being a recurring character appearing in every season except X/Y.
it didn't have anything to do with Charizard.