Why does Ash leave all his Pokémon (aside from Pikachu) at home whenever he starts a new journey?
Ash proclaimed that he wanted to start all over again. His only companion was to be Pikachu, who has been with him ever since his journey began. It is also meant to be an advertising tool to promote the new generation of Pokémon video games. When Ash went to Hoenn, only Pikachu went with him. When he went to Sinnoh, Pikachu came with, and Aipom was a stowaway. Aipom’s evolution was one of the new Generation IV Pokémon, and it was correctly assumed that Aipom would evolve.