Some of these third party developers are just being stubborn and stupid.
Most of them don't even want to bother with the Wii. For instance, Epic stated they weren't developing for the Wii because it was not powerful enough for the Unreal 3 Engine. Then just a few weeks ago they reveal that they're bringing UE3 to the iPhone. What the hell?
The developers that ARE working on the Wii aren't much better.
Just looking at the top 50 selling Wii games on this site, it's easy to see what sells: Party games, family games, arcade games, etc.
The biggest sellers tackle the potential of motion control and allows people to play together. This is why many bought Game party, Carnival Games, and Mario and Sonic. They thought these games would provide new experiences with motion control that they could play with their families and friends (that's why a lot of people bought the Wii in the first place).
Then we saw Sega pronounce this year that they were going to "crack" core gaming on the Wii. We saw about a half-dozen titles that were either on-rails, had lame stories, or had overly violent and profane content as the answer to that. Now they say they've given up scratching their heads as to why they failed.
EA pretty much did the same, even though they had great success with their fitness titles.