I really don't think most Wii gamers go "meh, this game is made by a third party, so I'm not gonna buy it." I'm pretty sure many of them don't even don't the concept of a third party. If it's a good enough game that appeals to their interest, it shouldn't matter whether or not Nintendo made the game. The fact is that Nintendo is simply the developer that "gets" the audience the most, and makes the most quality games that appeal to Wii gamers. I guess you could say there is also the factor of Nintendo having established IPs that are already known and loved, but at the same time, look at games like Wii Fit and Wii Sports - new ips but major hits.
If Wii Fit wasn't made by Nintendo but was the same quality, I'm pretty sure the game could have gotten near the same success. People bought Wii Fit because it looked like a fun, quality game that did something different, not because Nintendo made it. Most 3rd party Wii games just don't have these qualities.
Look at games like Just Dance, Carnival Games, Guitar Hero, etc. These are games made by third parties, yet they have sales in the millions.
