Why can’t I transfer all pokemon to let’s go?

Why can’t I transfer all pokemon to let’s go? Only the Pokémon you’ve caught in Pokémon GO that were originally discovered in the Kanto region (as well as their Alolan variants) may be transferred. You can’t transfer Pokémon from Pokémon: Let’s Go, Pikachu! or Pokémon: Let’s Go, Eevee! to Pokémon GO.

Only the Pokémon you’ve caught in Pokémon GO that were originally discovered in the Kanto region (as well as their Alolan variants) may be transferred. You can’t transfer Pokémon from Pokémon: Let’s Go, Pikachu! or Pokémon: Let’s Go, Eevee! to Pokémon GO.