At a guess, it's probably because big N has carved out its place in the current market with young gamers and gamers outside of the standard 18-34 year old male market. The very young aren't allowed to play Madworld, and a lot of the others fall outside the "Mature" market in other ways- people who just play the pack in titles and the occasional bargain offered at grocery stores, people who play titles like Wii Fit and Dance Dance Revolution for exercise, and so on. Where there's overlap in the Wii market with what most think of as the typical gamer market, you have a dedicated fan base that grew up with many of Nintendo's mainstays... And you also most likely have people with more than one games machine, and given a limited array of options, they'd probably rather play their "Hardcore" games in HD, or using a mouse and keyboard, or just with some controller other than the Wiimote.
It also is probably worth noting that the Wii has always been the low-price option, both regarding system hardware and the kind of television one needs to get the most out of said hardware. And the Wii games that have been pitched at the "hardcore" have generally been some of the most expensive available for the system, at least at launch.