Pokémon made it clear very early on that Ash's Pikachu wouldn't be evolving into a Raichu over the course of the series. While there have been reasons provided by Pikachu over the years for why it doesn't want to evolve, the truth may well be that, regardless of whether it wants to, Ash's Pikachu simply can't evolve.

Ash's Pikachu's desire not to evolve is first demonstrated in "Electric Shock Showdown," the 14th episode of the series, which saw Pikachu battling Lt. Surge's Raichu and getting beaten badly. Pikachu refused Ash's offer of a Thunder Stone, knocking it away while insisting that it wanted to win as a Pikachu, for all the Pikachu everywhere. Ash was able to make use of Pikachu's speed-based moves to ultimately outmaneuver Raichu, proving it didn't need to evolve to win. The idea would come up once or twice more, but Pikachu never changed its perspective, even into Pokémon Journeys.

Ash's Pikachu Has A Rare Ability That Prevents It From Evolving

Pikachu Has the Gigantamax Factor in Journeys

In the episode "The Climb to Be The Very Best!", Pikachu is doing battle with a Gigantamaxed Drednaw, which is wreaking havoc in the Galar region. Pikachu gets exposed to Galar particles as well, causing it to grow to an enormous size. However, it doesn't just Dynamax; Ash's Pikachu actually Gigantamaxes, taking on a chubby new form and gaining the ability to use G-Max Volt Crash. Pikachu eventually reverted to its ordinary size and appearance, but after being given a Dynamax Band, Ash became able to Gigantamax Pikachu whenever he was in a valid location.

This may not seem particularly important, but in Sword and Shield, Meowth, Pikachu, and Eevee with the Gigantamax Factor (i.e., the ability to take on the rare Gigantamax form) are actually unable to evolve so long as they have it. There is a way to remove the Gigantamax Factor, if so desired, but if Ash's Pikachu has had the Gigantamax Factor all along, then it was actually never even capable of evolving into Raichu. This does explain how Pikachu was able to come into physical with the Thunder Stone without evolving, answering what would otherwise be a plot hole.

Ash's Pikachu Isn't the Only Major Pokémon With the G-Max Factor

Team Rocket's Meowth Also Carries This Rare Trait

Later, in "Sword and Shield: From Here to Eternatus!", it's revealed that Team Rocket's Meowth also possessed the Gigantamax Factor. Meowth is transformed by exposure to Galar particles from Eternatus' Dynamax Cannon, and attempts to attack Eternatus in retaliation, but Eternatus ends up absorbing all the particles, reverting Meowth to its ordinary appearance. While Meowth has never expressed much interest in or objection to evolving, it would seem that he was similarly unable to evolve into a Persian all along.

Amusingly, this does mean that Ash's Pikachu was special all along, just like Team Rocket thought. It's not clear just how rare the Gigantamax Factor is, but Ash's Pikachu is the only Pikachu shown to have it in the anime, so it could be quite rare. Regardless, it offers a convenient and genius explanation for why Pikachu never wanted to evolve in Pokémon.