FPS on consoles is also a growth genre, so there was lots of room for more of them. Someday, it'll reach maturity (it's starting to get there now, I think), and there will only be room for Halos, Calls of Duty, etc. Wel-known "classic" (for lack of a better word) Nintendo franchises, on the other hand, occupy almost exclusively mature or post-mature genres, so there isn't a whole lot of interest in newcomers.
Ultimately, the real problem is that Nintendo seems to have simply run out of interesting ideas to keep Wii going. I don't think Wii Party's relatively low appeal is because the nobody wants new Wii x titles, but rather because Nintendo hasn't introduced anything compelling in the Wii franchise in a relatively long time. Wii Party has only itself to blame for being uninteresting.
Software drives the market. The Wii platform is still healthy, even if it's not pulling astronomical numbers. Interesting software would pull it to high heights once again.