Orre was, to make it blunt, just an outta the blue Region. It was overriden with desert, which suppossedly made it impossible for any Pokémon to live there. Then in XD were the Pokéspots, as wild Pokémon started to flow around. And yes, they were old ones. I have my own theories, but I better save them for my Fic.
That's the only excuse. Still, it could have had a few new Pokés, but again, it was just some excuse to put 2nd gen Pokémon somewhere else and save them the hassle of remaking Johto. Them japanese fellows must hate it...