Killzone 2 is not objectionably better than Halo 4. Halo 4 is being criticized more harshly that Killzone 2 was and is being held back on Metacritic by a few troll scores. If anything Halo 4 is a higher quality game than Killzone 2, but that's more of a taste thing since they both have good graphics and polished gameplay.
Perhaps Killzone as a franchise just doesn't appeal to people like Halo does. Maybe Killzone is too gray, brown, and gritty. Perhaps their enemies look to alike and the environments are too bleak. Maybe people like color and style and big open environments that let them attack from any angle. Maybe Halo sells so well because it just that good.