3rd parties hate the Wii because they have to change their approach. Last gen, when all competiting systems were more or less the same technically, making a hit game was easy: Make sure it looks better than its predecessor.
Today the market leader doesn't allow for doing it that way, so what to do? A lot of 3rd parties heavily rely on their games being cutting edge in the visual department. On the Wii, they can't. What's worse is that the technology is there (on ps3 and 360), so the level of expectations has already been raised - there's no way for them to impress with a Wii game, especially when they try doing multiplatform releases. In an industry based on "bigger is better", downgrading just isn't sexy.
Clever move on Nintendo's part... Multiplat doesn't work, because people won't buy a Wii game when it's just the ugly sibling of a shiny HD version. Since ignoring the market leader is not an option either, 3rd parties will have to increase their exclusive Wii support.
Currently playing: NSMB (Wii)
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