The fact that the Wii has 1/4th the main memory (and not anywhere near that of video RAM) of the HD consoles is really what prevents these kinds of titles from showing up on it.
Its easy to knock down the graphics a notch or three. Fitting a game that barely fits into 512 MB (256Main/256Video -- I know the X360 can draw the line elsewhere, but this is how it lines up for cross platform games, most likely), into 91 MB (64Main/27Video)... that's not so easy.
Dare I say... so hard that it demands the game change fundamentally, and thus the publishers won't do it, and likely Nintendo won't allow it, unless the game is nearly as good looking on the Wii as it is on the HD platforms. Nintendo doesn't want side-by-side comparisons if the other platforms are going to look better. I would wager heavily that shipping a game cross-platform and including the Wii means that you make the Wii look pretty dang good next to the HD consoles, or you lose the Wii deal.
