I think the answer is fairly simple.
America.
If you want to blow up in pop culture, you have to go over very well in America.
Gears and Halo are pop culture phenomenon.
I've not seen any KZ2 commercials, nobody(in general) over here has a PS3, and if they do, it's probably a status symbol.
You can't expect a game to go Halo 3, when America isn't excited about it. You just can't.
America is the reason those PS3 exclusives are underperforming. There ya go. Another reason to hate us. If one country is the entertainment capital of the world, we're it. Movies, music, TV, Internet, games to an extent. Entertainment is our number one export. You, for instance, are using American entertainment in the vast majority of websites you visit each day.
While doing well in the ever increasingly less important Japan, and the "entire rest of the world" we dub others, is also important; when you don't do well here, you'll never be a megaton.
Obviously.
I mean, who the hell do you think this game was aimed at? It certainly wasn't the UK or Japan.
We happen to be in a recession, and 460 dollars for a console, plus 60 dollars for a game is beyond the means of some of us working stiffs. Especially a game and company with poor marketing.