It would have sold better on 360 and PS3. Games like Uncharted and The Crackdown have both sold better on PS3 and 360 than similar games on the Wii. It's not a niche game as some have said and unlike Okami (an adventure game where the main play mechanic is painting) and Psychonauts (a platformer except for the big few the majority of platformers since the 32 and 64 bit gens have been rather unpopular) because gamers on the hd consoles love violent action games and it has the whole Sin City aesthetic going for it with a lead character that was very reminiscent of Sin City's Marve. And Sin City was a very popular movie. Like No More Heroes it would have sold over 1 million on the 360 or PS3.
Also, for all the talk I've seen some make. Madworld isn't a bad game. It received 90s from Play and IGN, and I believe its Metacritic score is still higher than Wii Sports, Wii Fit, Wii Music, Wii Play, and Mario Kart Wii.