I'm also sure that, especially after the Wii, parents (and even some gamers) are scared to buy non-Nintendo games on a Nintendo console because of the amount of utter shit we have seen in the past. At least with first party Nintendo games, apart from a few like Wii Music, you're going to get a decent title.
And there's just always something holding back a Nintendo version of a multi-plat release that I imagine makes it sell less. A lack of HD, scaled back graphics, missing online modes, that sort of thing.