Pikachu can’t progress unless it is offered a thunderstone. Ash has the stone with him; however, there are some possible factors that he is not developing Pikachu. As displayed in The Electric Shock Showdown, Ash’s Pikachu is happy to be a Pikachu and can beat a Raichu without progressing. It had declined to develop when Ash asked it to, stating that it would beat Raichu as a Pikachu.
It has beat Team Rocket thousands of times. It has hit Ground-type Pokemon (yep, I understand that’s an Anime Error, however still …) It has knocked famous Pokemon (Remember Regice and Latios?). In both the videogames and in the anime, Pikachu can just develop to Raichu by utilizing a Thunderstone. While at the Pokémon center, Ash thinks about getting a Thunderstone to progress Pikachu, to win the fight.
Ever since Ash has had no qualms developing other Pokémon for his gains, however never Pikachu. Numerous quality this to their close bond, however personally, I associate it to Ash being an utter idiot. Ash hasn’t progressed a single Pokemon utilizing a development stone over twenty years. He does not like requiring his Pokemon to progress.