Since the first episode aired in 1997, the Pokémon anime has featured a large number of Pokémon. The various evolutions that a Pokémon can go through are an important part of Ash's journey and mark significant events in the show. Evolving is central to the mechanics of the franchise because it is required to complete the Pokédex and remain competitive.
A well-known and beloved feature of Pokémon is 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. Some of Ash's Pokémon, however, 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.
Squirtle and Bulbasaur might have chosen not to evolve
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.
It has been questioned why Charmeleon and later Charizard refused to listen to Ash. Many people noted that if a traded Pokémon's level rises too much, they won't listen to their trainers. Technically speaking, Charmander wasn't traded. Damian, his previous trainer, simply got up and left him; no Pokémon, cash, or goods were exchanged.
Charizard's disobedience might have resulted from Ash's poor instructions and the fact that he didn't really earn his first few badges. But before, Ash had given strange orders that had been carried out. Several remarks on the Reddit thread mentioned that Charmander was mistreated by Damian, which may have had an adverse effect on how he evolved. Whatever the situation, Bulbasaur and Squirtle seem to be unwilling to evolve because they don't want to become like their ex-friend.
It's possible that Pikachu stopped the two Pokémon from evolving
However, as one commenter noted, Pikachu is another Pokémon that might have had an impact on this choice and others that followed. Pikachu is known for not evolving, initially out of spite for Lt. Surge's Raichu and later simply out of a desire to remain a Pikachu. Pikachu, however, is unquestionably one of Ash's strongest Pokémon and is capable of defeating Legendaries. It's possible that seeing Pikachu's power made Ash's other Pokémon believe that evolution is unnecessary.
Does Ash's team consider evolution to be important? Most likely not, given that Bulbasaur and Squirtle were two of Ash's top performers and had powerful moves like Solar Beam and Hydro Pump. Additionally, the fact that it is implied that this is the Pokémon's decision gives them more depth, a quality that propelled the franchise through its early years. These monsters are certainly cute and cool, but because they are also unique, fans remember them for a very long time. Not because they are two of the most well-known Pokémon in the series, but rather because they have personalities, Bulbasaur and Squirtle stand out.