Release date, I saw quite a few people buying them around the time they came out, a bunch of sports games sold I believe. So I'd say starting a year early is a good idea in America.
Price. By the time the PS3 hit the floor, it was not exactly a cheap purchase. Americans love perceived value.
Live. The only person I know that owns an xbox doesn't use it much. But I know many people that absolutely adore live and spend the majority of their days playing halo 3. Not saying all 360 owners play halo 3, but it seems to be a "popular" game.
Western appreciated titles. Lots of guns :P Hell even their most critically acclaimed RPG has guns xD They chose a good "theme" if you will when releasing titles, better sports games, more guns.
Advertisements. If I have to give MS credit for anything, it's the ability to advertise like no other :P I can't tell you how sick I got of seeing Gears of War advertisements, then I thought it could get no worse, but MS defeated my knowledge and showed me Halo 3 :P Americans are MASSIVELY influenced by advertisements, I believe more so than any other country xD We love our Celebrities, billboards, and commercials :P America is a Mass Media whore of a country :3
I believe the PS3 will gain a lot of rep with a few of it's big hitters showing up this year, plus now that it seems Blu-ray is going to be accepted as the new high def standard, tacked on with more HD-TV's being sold and PSN getting a bit more to offer always helps. I don't see it being a massive 180 in America, but I see the market getting more passion about the PS3 within the next year or two. I expect sales to be about the same at the end of this Gen.