I know it's an unpopular opinion in here, but I still feel it also has a lot to do with image.
I believe Wii has an image that is intimately connected to Wii Sports. It's fun and accessible. When people buy a Wii they do NOT buy it because they want deep gameplay, a serious attitude or an epic story, but because it offers a fun and new experience. Something to enjoy with friends. (System sellers: Wii Sports, Wii Fit, Mario Kart Wii, Wii Sports Resort)
When people buy a PS360 this is not the case. Online gaming, epic battles, serious stories and state-of-the-art graphics are the reasons people get the HD consoles (System sellers: Halo 3, Grand Theft Auto IV, Metal Gear Solid 4).
Companies really have a hard time to relate to this. They struggle to find the blend of serious attitude and accessible gaming that will appeal to the Wii audience. Most of the 'hardcore' games released has not yet caught the imagination of the person who bought the Wii Sports Console. They are simply unappealing to the audience.
Naturally, I'm generalizing here. There are plenty of people that do not fit this stereotype. I also need to point out that Okami sold better on the Wii than on the PS2, that most so called 'hardcore' games on the Wii have been halfassed at best and that third party support for Wii has been spotty.
I know this theory is politically incorrect. It just appeals to me with a theory that assumes people know what they want instead of them being stupid and having bad taste.