People just want to see games that they like be successful so more games like that are made.
For example: I am sorry to see GTA:CW sell so poorly (so far) as it's a really top notch effort and I really enjoy the game. But then the same goes for Dewy's Adventure. I'd love a sequel to that but it'll never happen due to poor sales.
I do find the game media focuses too much on M games and while I do plan to buy both HoTD:Overkill and MadWorld, I don't want them to be big hits because the last thing I want to see is Wii filled with shallow games with a ton of swearing and excess violence. While MadWorld may have some depth, it's copycats likely won't.