Like the two above me said, it only moved about 70,000 copies. Less than half of what they shipped. And to top it all off, it's on the highest-selling home console of this generation.
Wasn't it being built up as being one of the "big" "core" gamer games of the year for the Wii? Just like Madworld before it, which also sold below expectations. If these games are what "core" gamers on Wii want to play, why aren't they selling that well? Likely answer: Because they've given up and moved on to a different system.
The games got mediocre to bad reviews. Core gamers are not going to spend 50 bucks on poor games. It's sad that these poor, low budget, shovelwere games (which shamelessly wrap themselves in the "core gamer" blanket) are used as the litmus test for future Wii core game fundings. Look at what THQ did with Deadly Creatures. The developer threatened the Wii core gamebase to buy their lousy game (one of the worst Wii game I had the displeasure of playing recently) or else they'll go back to make only minigames for the Wii -- not to mention the fact the game is nearly unmarketable (game where you control a creepy spider and a scorpion? It would not sell on any system even if it's AAA quality), and THQ knows that since they literally did not market the game at all.
At least MadWorld sold more than its ps2 God Hand brethren. Considering the skeleton crew that made this game, can they really call the sales figure disappointing? They probably made large profit already.