I say that it's simply the lack of MAINSTREAM hype (and no, I'm not saying mainstream with contempt, like some college professor or indie musician).
Sure LBP had a lot of hype, but aside from the UK who saw LBP on CNN, Soda Cans, or TV (I only saw the very short, vague, and stupid commercial with the kid looking at reflections outside of a window).
Here in the US, on the East Coast, I saw E-Trade commercials with a "hip" young investor saying he bought a bunch of tv screens to keep track of stocks... and own his friends on Gears of War.
We saw with the infamous thread about which game had more hype (KZ2 vs. Halo 3) that Halo 3 obviously had more hype, both on the internet and off.
All of those games, with the exception of Fable 2, are considered part of mainstream American pop culture. They're a "must get" as Call of Duty is.
Look at GTA 4, controversy and news coverage only helps that game sell, rather than hinder.
The fact is that aside from hard core fans, nobody knows about Killzone 2.
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I want all PS3 owners to do this. Go to someone you know who owns a PS3, but you don't consider "hard core". Ask them what games they have. Most likely they will have Call of Duty, GTA IV, +sports games.
So does that mean PS3 owners are simply casual gamers? No, as they have bought "hard core" titles such as CoD and GTA. The only differences between COD and GTA vs KZ2 and Resistance 2 being that one side is known by fucking parents, who will buy the games for their kids, and the other being raved and ripped apart by fan boys.
We hardcore gamers are the minority, and Sony needs to realize that. They need to really shape up their marketing. While it was encouraging to see the KZ2 tv ad (which I didn't see on tv...), that alone won't help.
They need to go beyond the hard core fans, and if they truly want to move hardware sales, beyond the user base, and make people look at the PS3 and its game.