LogFAQs > #930133726

Honestly, insisting on having the full National Dex catchable in every single Pokemon game has been more of a problem than anything else, given how large it's gotten and what that means for balancing and having a competitive metagame (particularly where so many pokemon that are available aren't available without bringing them in from a previous generation). Having "only" 450 or whatever in the game is still more to catch than any game in the first 3 generations had, while being a much more manageable number for balancing purposes and ensuring people can access the competitive metagame without also having to buy the last three games to get older Pokemon that haven't been succeeded by ones that are available in this game. It's just more practical to have a smaller (but still quite large) number to work with.

Sure, it seems like a step backwards because we're so used to the National Dex, but really, the National Dex has been a mistake since its inception. Older Pokemon should only ever have shown up in later games occasionally, as nostalgic cameos. Trying to keep every Pokemon in every game was only ever going to result in impractically large numbers. This change was inevitable.