The game is built heavily on the BW engine. Releasing it as a 3DS game is not a move for the fans or Nintendo. Releasing on DS makes the game available to more people at a lower priceand will result in higher sales for Nintendo. 140+ million DS systems sold means way more Pokemon fans who haven't yet moved to the 3DS.
Releasing BW2 on 3DS would just be a means of pushing for 3DS sales. Remakes and 3rd versions are not the way to move Pokmon to the next generation handheld. I don't understand why people think it would be a good move now. DS generation started with Diamond and Pearl. GBA generation started with Ruby and Sapphire. GBC with Gold and Silver. Gameboy with Red and Blue. There's no reason to thin the 3DS generation would start at a remake or even less likely a third version.
If the game engine is built for the DS and the game can run on the DS, it would be stupid to release it as a 3DS exclusive for wider screen resolution, spotpass, and streetpass. --- Gutalala Sudalala ONE PIECE, IT EXISTS! - The Great Pirate, Edward Newgate "Whitebeard"