It's all marketing and fanbase based.
Gran Turismo franchise sells well because it has a past history of fans and a great fanbase on it's consoles. When Gran Turismo 5 is released. It will easily sell 4 - 5 million no problems.
God of War franchise sells well (Given the type of game it is) it has a core group of fans. When God of War III is released, I think it will easily sell 2-3 million.
Comparing Kill Zone 2 to Halo 3 / Gears of War 2 in terms of sales is a bit of a fallacy. Kill Zone is the sequel to a game many people have never played and many of the people that DID play it, Didn't like it or found it to be a BAD game.
Halo 3 is the sequel to two very successful predecessors. Gears of War 2, sequel to a successful original game.
Unfortunately, we can't see the original Halo's numbers BUT I guarantee you Halo didn't fly off the shelves (Atleast initially).
Through two weeks of sales the original Gears of War had only sold :
Week 1 910,931
Week 2 465,778
A total of 1,376,709 copies.
Kill Zone 2 has sold comparable amounts.
I bet Kill Zone 2 grows some legs and ends up selling fairly well and will be a solid selling franchise.