That's how it has always been for Ash's Pokémon though. Charmander evolved twice while Squirtle stayed the way he was. It's a recurring theme in the show: Pokémon evolve when they want to evolve, and it's considered ethically wrong to force an evolution.
In addition, western Pokémon fans may prefer the big tough Pokémon, but the Japanese Pokémon fans prefer the small cute ones. (Munna is VERY popular in Japan, for instance. I don't hear nearly as much about Munna in America.) The anime writers want to fall into a middle ground to please both audiences, so the cast members have a mix of both.
(Did Bayleef ever evolve into Meganium?)