Pokémon first ventured out of the Gameboy videogames as a comic book and not a TV show. In the comic book, Clefairy was chosen as the main Pokémon character, in order to make the series "more engaging" (translation: Bulbasaur, Charmander and Squirtle have no personality).
However, when the series was ready for a TV adaptation, the marketers decided that Clefairy wasn't cuddly or yellow enough. They replaced it with Pikachu, who they thought could be identified more easily by younger viewers. In fact, the only other yellow and cuddly character on TV was Winnie the Pooh. Interestingly, both Pokémon can only evolve using a special stone.