the Unown may've counted as more Pokemon. Though, even then, the number still pales compared to Kantou and Houen's. I'd go with the theory that they were just pressured for game space (GSC supposedly pushed the limits of the Game Boy Color enough as is). Sure, several Kantou Pokemon were available (though, I really wish they would have given more reason for the backwards compatibility), but all things considered, the data for all the Pokemon was there anyway. If the data wasn't there, all your favorites from the old days would be unavailable, even with trading. I'm pretty sure it takes up less data to program where an existing monster will appear in an area than it takes for the monster data itself, so most of the Kantou Pokemon being available in the game doesn't seem like it'd have much to do with the lack of new Pokemon.