Why exactly is it impossible to add new pokemon to newest, gen 6-7 roms?

I know the hacking of those generations is relatively new, compared to gen 3, duh. But I've heard a lot of times that it's simply impossible to add new pokemon to the game, like more shiny forms or, I dunno, delta forms or something. What exactly makes it so hard to do that? How is the rom structured that no one so far figured out how to do that?

Bump, because I'm incredibly curious and want to overcome this problem. Any help?