Released in the US in 1998, Pokémon has become a pervasive smash hit. It's almost impossible to find someone who hasn't heard of the series or video games at this point. The original season was beloved by many fans, and some still continue to return to the classic "Indigo League" arc. Though the season is over two-decades old, it sets the standard for every following installment. One such component that has remained consistent throughout the series is evolution.
Evolving is at the core of the franchise's mechanics because it's required to complete the Pokédex and remain competitive. It's a well-known and beloved aspect of Pokémon that if a trainer takes good care of their partner by battling with them, giving them a specific item or meeting some other criteria, a multi-stage Pokémon will naturally evolve. However, some of Ash's Pokémon do not evolve in the show. Specifically, out of the first generation's three starter Pokémon -- Charmander, Bulbasaur and Squirtle -- only Ash's Charmander evolves, though all three can.
One Reddit user theorized that the reason Bulbasaur and Squirtle chose not to evolve is because of Charizard. As a Charmander, it was clear to see that they were friends -- the episode "Island of the Giant Pokémon" is given as an example of this friendship. However, once Charmander evolves, he loses his previously friendly attitude, frequently disobeying orders, sleeping on the job and ignoring and attacking Ash. The fan argues that, upon seeing this drastic change in their friend's personality, Bulbasaur and Squirtle were so horrified that they swore never to evolve, lest they end up the same way.
Now why Charmeleon and then Charizard refused to heed Ash has been debated. Several people pointed out that traded Pokémon won't listen to their trainers if their level gets too high. However, Charmander technically wasn't traded. Damian, his old trainer, just up and abandoned him -- there was no exchange of Pokémon, money or goods.
A YouTuber once pointed out that Charizard's disobedience could have stemmed from poor commands given by Ash combined with him not really earning his first few badges. Yet, Ash had given odd orders before that were obeyed. Several comments on the Reddit thread pointed out that Charmander was abused by Damian, which could have resulted in him being negatively affected by such trauma as he evolved. Whatever the context, not wanting to end up like their former friend seems like a solid reason for Bulbasaur and Squirtle not to evolve.
But there's another Pokémon that could have influenced this decision and affected the choices of several Pokémon afterwards, as one commenter pointed out: Pikachu. Pikachu famously never evolves, firstly to spite Lt. Surge's Raichu and then just out of a desire to stay a Pikachu. And yet, Pikachu is easily one of Ash's strongest Pokémon, able to take on Legendaries and win. Seeing Pikachu's strength might have prompted Ash's other Pokémon to think evolution is superfluous.
Does evolution really matter for Ash's team? Considering Bulbasaur and Squirtle were two of Ash's powerhouses and possessed strong moves such as Solar Beam and Hydro Pump, probably not. Additionally, the fact that this is implied to be the choice of the Pokémon adds depth to them -- a feature which carried the franchise through its earliest years. Yes, these are cute and cool monsters, but they're also individuals, which keeps them in fans' minds for decades. Bulbasaur and Squirtle are memorable, not because they're two of the most famous Pokémon in the franchise, but because they have character.
