So the important factors in choosing a platform are:
1) ease of development
2) market penetration
3) team members’ existing skill sets
4) portability of code to a given platform
5) acquisition costs of development kits and materials
I'd say that Nintendo has 1, 2, and 5, but probably turns a lot of third-party developers off due to 3 and especially 4. Most of the Vudu-approved "developers that matter" have skillsets suited to developing PC-style games, which makes them (and their companies) perfectly comfortable with developing for PC/360/PS3. They're good at that, so why do something different like a Wii game, that might completely tank in the marketplace? It makes sense - business sense - to play to your strengths.
Developing games for Wii also shackles a developer to only being able to use that code for future Wii games. In other words, it's not like 360 code that can be ported to PC and PS3 in a reasonable amount of time to make some extra cash. Take The Grinder for instance...HVS had to license a game engine for the HD platform work, since (apparently) none of the work they did on the Wii engine could be used elsewhere. It must be tough for third-parties to hitch their wagon exclusively to the Wii, when there's no guarantee that their non-Mario/Zelda/Kart/Metroid/Sports/Fit game will even sell.
Besides, it's not like Nintendo cares about these third-parties anyways, really. Unlike Microsoft and Sony, who need third-party royalties to mitigate the losses they take on their hardware, Nintendo doesn't need to cater to third-parties because it makes money on its own software AND hardware. Third-party sales are gravy to them, so it's very easy for them to sit back and shake their heads, telling third-party developers that they just don't "get it".
I guess third-parties are taking that to heart, and aren't trying to "get it" any more.