>>24911848He's the cute face of this game
whose smile we want to protect, a new pokémon so people obviously never picked him before, and the partner's personality fits him so fucking well.
He's not OP but is pretty good for having Psybeam, but he was already popular in voting polls before the game came out so it's not really that. If anything Chespin is the "OP" starter for having an amazing movepool, while Fennekin is more limited.