Look, you have judge this fair. In its simplest terms, GoW3 belongs to a niche gaming genre right now. If you look at all the games that are in its genre, you would see that God of War 3 is selling head and shoulders over them. Bayonette, Dante's Inferno, Ninja Gaiden 2 + Sigma, and its closing in on Devil May Cry 4.
It was never going sale better than AAA shooter or a game with Mario in the title, but it is selling well enough to say that its one of the best games on any platform for its genre.