Games like Mad World and Dead Space Extraction probably fall well outside what the majority of Wii owners are looking for in games, whereas something like Carnival Games, despite being inferior, does not. Not to say they CAN'T sell. I mean, HotD: Overkill sold over 500,000 copies (I think it was luck, really). But doing things the old way isn't gonna cut a sizable profit on the Wii. PS3 and 360 are much "safer" in that regard, because third parties don't need to shake up their marketing formula much and sales are much more predictable.
Something I've sorta noticed about successful Wii games is that their commercials (if they have them) or trailer feature people playing the game more than the actual game itself. Sure, they'll show what is being played, even explain it a little, but the focus is on how much FUN people are having with the game, rather than how awesome the game appears to be.
However, to make such ads for single-player games is a difficult task.