There are three stages of discovery as a person plays Pokemon

1. Discovering that there are Pokemon other than Pikachu

With the television show, the video game, card game, and comics featuring Pikachu as a protagonist, it is often a bit of an epiphany for a person to realize that they have a degree of freedom with their favorite Pokemon, even if they discover this immediately when they turn on the game or open a pack of cards. This epiphany is especially strong for younger players, since age-appropriate media almost uniformly features Pikachu, and it's a bit of a leap of brain processes when they realize that even though Nintendo wants them to like Pikachu, they can like something else instead.

For some people, realizing that they can like other Pokemon or even just like a bunch of Pokemon leads them towards exploring these options, and some people even end up preferring them. Pokemon allows a degree of freedom that other games don't provide, and being able to choose from a wide variety of creatures is appealing to many people.