There's the stereotype that Americans think that America equals the world. It's becoming increasingly more likely that Microsoft holds this belief.
One of my theories aboiut the 360 success being very regional in its strongholds is that Live is very much part of what made the engine go and gave MS its foothold early on. We've seen how in other than the really big evergreen Live games (Halo, COD, Gears, the big 3 sports games, etc.) the audiences for online titles tend to peak early and die off quickly except for the hardcore fans of other titles. We've all had trouble in older titles outside of the top games trying to get matches to play or get achievements or whatever. When you factor in the added piece of language barriers, Live loses its luster if you're trying to get matches for titles in any non-English speaking zone. So it makes sense that even outside of America, the places that 360 does best is other English-speakign countries. Still ahead nicely in the UK, pretty dead even in Australia, etc.
Add to that equation that the titles that 360 has for exclusives tend to be very Western-centric games, especially after their foray into Japan failed early in the gen for a bunch of reasons (but stil primarily dominance of handhelds, the relative lack of onterest in online play at the start of the gen especially there, and that the other two console-makers are homegrown products), and you can see why the 360's appeal is so centered in certain areas.
Until a console reaches a certan critical mass with enough players playing in a certain region/language AND has enough of that market playing certain titles at the same time, MS (or anyone else) will struggle to find footholds if they're pushing connectivity and co-op/party/etc. as primary selling points for a console. It's sort of the same reason an MMO might succeed in one region and not another. If I can;t find enough players playing at my peak hours in the Central U.S. timezone of an MMO, the game doesn't work well for me and I'll probably bail for WoW or something that has the critical mass needed for me to fully enjoy the game. Like if I wanted to play an awesome MMO that's huge in, say, Korea, but I can't find enough English speakers to pay with, I'm kind of screwed if I don't speak fluent Korean. It's a bit of that on a bigger scale, reversed.
Once the U.S. had a huge base/lead on PS3, it's hard for them to catch up, because people want to play with their friends, and we hear it all the time from people that may have wanted a PS3 or to play COD on PS3 or whatever, but buy 360 or 360 versions for online play because that's what their friends have.
Anyway. Hope that made sense. It's late and I had beers.