Well, part of the problem is the mentality that Sony has around this game. It's a mentality that dates back to Halo. The original KZ was made because of Halo. At the time when Halo came out, it was a unique experience on consoles because it worked so well. People wanted to be a part of that and it sold droves of consoles and Sony wanted a part of that too (oops).
Now the market is saturated with FPS games and a lot of them are really good. To move a lot of hardware, an fps would have to truly do something different, which despite fanboy rhetoric, KZ2 does not. Sony is several years late to the party on this one.
I'm convinced that no FPS will ever be a huge system seller ever again unless it is truly revolutionary.
"Man is born free but is everywhere in chains" - Rousseau
