Wikipedia was created and is hosted in the US, the Wikimedia Foundation is based in the US, Jimbo Wales is American, and I'd wager a large part of en.wp visitors and editors are American. Like it or not, Wikipedia itself is US-centric.

And though it may be besides the point, you know as well as I do that if SOPA happened to pass in the US it wouldn't be long before most Western countries had their own version too.