Because Digimon was a tamagochi clone first and everything else second. There was never a Digimon game formula or a Digimon card game formula or a Digimon anime formula. They just made random media to sell toys without any care for consistency. That lack of consistency made it have less general appeal.

Even today it's hard to introduce people to Digimon because there really is no right starting point, it's all different media and how much you like each one depends on your personal tastes.

Pokémon had it all consistent from day one. The tcg, the games, the anime, it's basically all the same even today. You can introduce someone to Pokémon with any media because it's all so consistently transversal.