I don't think the problem is that third party games don't sell on Wii, it's that good third party games don't sell as well as they should. Cheap shovelware makes a better profit on the system selling only a little less than big budget titles that bleed out at the bottom of the charts.
How many times do we have to say "why didn't 'game X' sell moar!?!" before people realize that there is, in fact, a trend, and not some vast HD-Wing conspiracy?