Microsoft is really good at giving the Xbox brand "perceived value" similar to what Apple does for their products. By that I mean, people think Xbox is the "trendy" console. Most people I know have one "because my friends do".
It's no secret that I dislike both the 360 and Microsoft, but i'm not even talking about the QUALITY of the 360 vs the PS3. UNFORTUNATELY most "gamers" in the US only care about what is popular atm. Most of the people I know who SAY they prefer the 360 barely know the difference between a multiplat and an exclusive game. They just assume the 360 is better because it's more heavily advertised and they are too lazy to actually go take a look at what Sony or Nintendo offer.
Before anyone gets pissed off, i'm not even talking about real gamers. I'm talking about casuals, and to be honest Microsoft does a better job of, convincing, casuals that the Xbox brand is the best.
Basically, people in the US are superficial and simply jump on whatever is popular to stay socially relevant. Microsoft targets those people more than Sony does.
Also i'm from the US, and my opinions come from what I notice about people here. Even some of my very close friends are like that :/
That being said, this time Sony isn't launching a year later or at a much higher price. It also won't have to play catch up with online features (the important ones anyway,lol). I think Sony will have an easier time worldwide and even if it loses the US, I see the gap being much smaller.
