Fanboys: Because Wii isn't superior and is inferior
Developers: Because only Nintendo games and casual titles sell so why bother
Sensible Person: Because developers are either still confused by Wii or aren't comfortable with motion controls and feel 'safe' with HD so don't 'go the extra mile' so to speak. Plus, due to a lack of marketing, lack of faith from retailers (as in ordering a plentiful supply and in-store promotion and bias towards casual titles/HD) has caused core titles to slump at retail so publishers/devs think: "that didn't sell very well, they don't like it, we'll put all our effort on the HD version". The userbase is also skewed toward the casual user who is unconcerned with superiority so publishers/devs think, the audience won't appreciate it so we'll concentrate on making HD as good. This doesn't leave much for Wii obviously.
