Limitations of the first gen, retained in FR/LG because they were too afraid to implement change of any sort to the original then. The pokeworld isn't that closed off, they know of all that's in the other regions. Why do they each have their own compendium of pokes that's ordered differently? A better question to ask is why said lists contain what's in the Safari Zone or other similar places where it's clear the pokes are not native to the region. On that, I don't think it was ever explained how second gen pokes are suddenly inhabiting Kanto. (or how pokes from other gens suddenly appear in the region postgame) There is only one conclusion: the dex itself is a fabrication of the game. If there is a compendium of pokes in the pokeworld, it would've never been organized this way, and certainly not be dependent on the region. Even if there are separate lists, they still would not look like what we've seen, since there are pokes living in the region that it does not list. It would be easier to just list all the places the poke lives than to limit the thinking of it just being in one region just because it was symbolic of the gen that introduced said poke.
