Headcanon likes to believe Red is responsible for all the pikachu pandering. well the real reason is to capitalise on the anime, but when it comes to an In-game reason, i like to believe red himself was responsible for that.

After Red take down all the gym, an entire mafia, and become champion with a pikachu when he was just 10 years old, the entire world got shook what just happen, and now everyone's capitalising on that event.

Like all the pikachu related content, within the games, like the pikachu park in ultra sun and ultra moon, how gigantamax pikachu exist, and just overall the canon implication that pikachu is a famous pokemon in-game.

that also means red accidently created mimikyu.