It makes absolute sense for GameFreak to showcase Gen I stuff so much. People who are fans of, say, Gen V and VI Pokémon and who'd like to see those Pokémon get alternate forms are already likely to buy the game. They've stuck with the franchise this long, after all.
The reason they stick with Gen I getting extra stuff is to draw in those who jumped ship after the massive hype bubble popped around the turn of the century, where a lot of fans jumped ship or didn't stick with the series. Well, now those kids who were fans of the Kanto era are at the age where nostalgia is big, along with the general push toward nostalgia in the media in general being big these days. So they try to draw them in with things that are familiar, but just a bit different.
It makes absolute sense from a marketing perspective.