But for the overall feeling like many prominent 3rd parties aren't supporting the Wii nearly as well as they should, I think it's because the Wii is such a clear change for gaming "philosophy." Nintendo's little white box, more clearly than many of their previous systems, epitomizes innovation over excess.
I dunno, surely there are devs who are so desperate to create the next crysis that they'd have a breakdown even considering the wii ... but I don't think everybody's like that.
It's perfectly possible to make huge, technically excellent, and "conventional" games on the wii -- just look at RE4! Sure the models have to be a bit lower poly, and the sweat might not glisten quite as brightly, but those are the sort of restrictions that game developers have always worked under.
Look at the PS2 -- it was notably inferior to the other systems (PS2 versions of games pretty much universally looked worse), but because of its popularity, the game companies bit the bullet and did what they could, and the result was that the PS2 had lots of excellent games, despite its hardware inferiority.
Another example is many PC games, where it's very common to support a wide range of hardware by gracefully degrading the graphics at runtime. Doing it at
runtime is hard work, but merely supporting different platform ports should be far easier, given sufficient planning.
Examples like those are why I don't really buy the "whining devs" excuse. I think the problem falls squarely on the shoulders of the people doing the planning, and that's management.