It would have also cost more to make in HD and it would have been competing in a much more saturated market on the HD consoles. On the Wii, Madworld is unique. On the HD consoles, it's just another blood-soaked button-masher. They chose not to advertise it, they chose to only make it a couple hours long, and they chose to basically make it a puzzle game in which you try to score X amount of points as quickly as you can to unlock a boss fight. That's the whole game.
I loved the visuals and the boss battles, but outside of the boss battles, there were really only 2 or 3 things that you did over and over for a couple of hours. Considering that the game was too short, too repetitive, and not advertised at all, I think selling half a million copies isn't even that bad.
They should have made the game even SHORTER, and sold it for $15. Then when people bought it, they'd go online and say "Hey that was pretty awesome for only $15," instead of "Biggest waste of $50 all year." The game was fun, but it actually had negative word of mouth.