While the exact answer is unknown, the reason is probably
Because it's Pikachu.
Pikachu is, in the eyes of many, the most iconic Pokémon. It make sense, because he's the mascot. Pikachu gets a lot of special treatment (getting in Pokkén twice, having its own game, etc.) and the anime cry for Pikachu is something that can connect with Pokémon fans of all levels.