Blastoise-shellshocker: TRR was released right after the FRLG expansion, and based off the FRLG video games (i.e. using artwork etc. from those two new games). That's why it had only Kanto Pokémon. As to why it didn't have Charizard, Venusaur and Blastoise as Dark Pokémon - no idea. Maybe because they got a Dark version way back in the original TR set? No good explanation, but that's the best I can come up with. :|