I think the biggest reason is that Pikachu was Ash's partner in the animated series, and that's probably what caused it to be such a success and endearing and widespread. But why was Pikachu chosen as the partner for Ash in the animated series? If you remember back in the original games, the first forest you go into, Pikachu was a very rare Pokemon there. It was a situation where, in places like an elementary school class, maybe two or three kids would have encountered Pikachu. The rest of them hadn't seen him before. It was a cute Pokemon, hard to get, and other students would wonder "How did you get that?" So Pikachu got a lot of buzz, but the internet wasn't around then like it is today; it had to spread by word of mouth. It was a rumored Pokemon that existed in this forest.
That was one of the reasons we chose it as the partner for Ash in the animated series. We wanted to feature it because of the rumors that spread about it. The guy who originally coded that part of the the forest, he liked Pikachu because it was a cute Pokemon. He just wanted to catch it for himself, so he made it very low encounter rate. Extremely rare.
Though I guess he wasn't always as cute as he is now [laughs]."
- Junchi Masuda
