The reason why 3rd party Wii games sell badly is because they are either too weird and different or a in a niche genre.
Think about it.
Madworld had a strange visual style and was ultra-vioent. Games with strange visual styles usually don't sell well.
HoTD Overkill, Extraction and Darkside Chronicles are all Light Gun shooters, which don't usually light up the charts anyway. They're fairly short games. People don't want to pay full price for a really short game, even if it's got high replayability.
I don't want Nintendo to bring out a new handheld, (unless it's a new Gameboy that only plays 2D game) because the DS is still going incredibly well, and it could keep going with it's hardware for another 2-3 years.