The most reasonable answer I can come up with is that publishers are trying not to develop a following for certain IPs and Genres on the Wii because they don’t want to splinter the market because the development costs on the HD consoles are so high the failure of one or two HD games could bankrupt some companies. The main reason I think this is because the DS (and often PSP and iPhone) receive versions of these games even though they generally sell terribly; and the Wii has demonstrated that it will reliably sell a million copies of a decent game with a well known IP, in a popular genre, if it is marketed moderately well; which is true of most of the games that publishers avoid releasing to the Wii.
Basically, many publishers (probably) believe that if they made a good version of some of these games for the Wii then they would sell 5 million copies across 3 platforms and 2 versions of the game rather than selling 5 million copies of the game across 2 platforms and one version of the game.
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