A game with barely any marketing on the expensive console didn't sell good.
I'm not surprised. Gears of War 2 sold 2.1 million in one day.
The PS3 needs a price cut, Resistance 2, LittleBigPlanet, and MotorStorm all have terrible sales. Games on 360 sell better because of the 360's low price. With the bad economy, not many are able to afford a PS3.
Hopefully that price cut in March is effective.