I dunno why people are being so negative about Black and White 2, they're doing something new, and a whole lot better than just a third version with a little bit more of content and a couple of story modifications, they're making completely new games with completely new plots, the only similar thing will probably be the region and Pokemon, it gives you more bang for your buck. And the DS platform choice is a good one, they're not alienating all the DS users, while 3DS users can still play and enjoy the games on it anyway because it's backwards compatible.
Because of the massive amounts of development time and effort that would take.
I'd imagine it would take out a fair amount of space out of the cartridge aswell.