It would just take too long to code them all in. They can’t delay the game because so many other media forms rely on the release of the game (i.e the anime, the TCG, the manga). It’s the same reason why they didn’t put all the moves in the game. The DLCs also take up a lot of room because they also have to code the releases in, as well as all the content in. Had they simply been in the game from the start (which they should have been in my opinion) then perhaps they would’ve had more room to fit more Pokémon and moves into the code.