Part of the reason i think is because the 360 came out eairler, and it debuted a lot cheaper than PS3, and its still cheaper than PS3. the other reason is Halo, the series is quintessentially American, now the PS3 has also got some American type heros, like Nathan Drake, but the games like Gears or Halo are just a lot stronger in the US market, possibly like Rolstoppable said, as an act of patriotism.
despite the failure rate of the 360 or the amount of peripherals, like batteries, or wifi adapters and the cost of Live the US market still prefers it over the PS3. however @ the OP next gen wont be as easy for microsoft as this gen imo. partly for those reasons but also because sony will likely release at the same price or even less if they have to.
i think kinect could also be a double edged sword for microsoft further down the line.
