Killzone would likely never sell like Halo. Killzone is deep, political, and morally complex. Instead of aliens from a Heinlein novel, the villains in Killzone were humans that the rest of humanity basically screwed over. The good guys, the ISA, are good in that they are the most similar to us.
Even beyond that, Guerrilla makes it even harder to differentiate good and bad; even if we wanted to be sympathetic to the Helghast, the brutal vengence-seekers they've become once more pushes them around to bad-guy status. Guerrilla sought to make this closer to the actual grim reality of war, where both sides do bad things, where both sides are controlled by power-mad leaders with goals they hold higher than lives.
The Killzone universe reminds me of Dune in a lot of ways, with political figures playing chess with entire populations. The Helghast are kind of like the Sardaukar, made stronger by the harshness of their world.
The atmosphere and story of Killzone are fantastic but I'm not sure it would catch on with the masses who just want to be a good guy and kill bad guys. Personally, I loved how KZ3 ended because it got me hyped for what could happen in KZ4. I hope it starts off with a major bang. I'm also looking forward to fighting the ISA in the Vita KZ game.