Because it's not possible to put a MW2, or AC2, or Borderlands on the Wii by just downgrading the graphics. Simply not possible - if it were, then why wouldn't anyone have done it yet? Capcom would have undoubtedly loved the ability to put RE5 on the Wii for a minimal budget, but it ain't that simple.
The only way it'd be possible is to get a seperate team to work on these Wii versions of games from the ground up, like what typically happens with the movie licensed software and most games that appear on a range of consoles. Companies just don't have the resources to do that, I guess, among other reasons.