Why Doesn't Ash's Bulbasaur and Squirtle 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.

Why hasn't Ash's Squirtle evolved?

Ash originally wanted a Squirtle to be his starter Pokémon, but Professor Oak told him it was taken by a Trainer who was not late. Squirtle is the only member of Ash's original team of six (Pikachu, Butterfree, Pidgeot, Bulbasaur, Charizard, and itself) that has neither evolved nor ever explicitly refused to evolve.

Does Ash's Bulbasaur not want to evolve?

While all other Bulbasaur evolved into Ivysaur, Ash's Bulbasaur refused to do so, despite facing the mass of Ivysaur as well as a Venusaur, all of whom were angry at it for ruining the festival. Ash then intervened and stood up for Bulbasaur and its decision not to evolve.