I am assuming that when Mii Channel development was finished, the "palette," so to speak, was finalized. All Wii games all need to contain the information for rendering Mii characters (it doesn't load things like textures or models from the Wii System Memory, only the necessary information), so adding more features to the Mii Channel just wouldn't work, because it would no longer be compatible with the palette in the Wii software. So, if a 3DS Mii was to be loaded to a Wii and then into a game, the game might crash because it has no data to render the Mii character from the palette it was originally given. Of course, this is just a guess, so...