There's a lot of contributing factors, part are the game's fault, part are the audience
The game is not mainstream on any level at all. Not on a Mario Kart level nor a Halo level. It's extreme violence with tons of swearing and a style that is far from the norm. It'll never have mass appeal. It is definitely niche. Also new IP.
Having said that I think there is a market for this on Wii.
The Wii is a family console and most owners have wives. kids or are themselves women. There's no way this is getting played in front of the women and children (some women will be exceptions and may love it but we're talking avg non-gamer Wii Fit wife here). Most families have a budget and the misses is having a say on which game gets bought. Mr Wii owner may want MadWorld but he may have to wait for his birthday to get this game that only he will ever play (and chances are it won't be the gift his wife or kids bought for him, hope he was a brother). Also Mr Wii may decide to get a game that he can play anytime instead of one where he has to wait for everyone else to go to bed first. Lastly, it's expensive. $55 (Cdn) for a 6 hour game with little replay value and severe restrictions on when it can be play and only daddy wants it vs buy a game like Mario Kart that everyone will play all the time, or 2 $30 games that everyone can play all the time, or 3 $20 games....
You start to see the problem. There is a market for it. Some guys are desperate for something more manly on their Wii, but if it's a shared console (and most are) he'll likely have to wait to get it. It'll become a much easier sell to the wife at $30.
In the meantime, House of the Dead will have to do and she can play too.
Remember - members of this site are not atypical Wii owners.