I've thought about this one for years.
My theory is that, for the most part, especially those outside of major cities, Americans don't have to worry about other countries. Who is on our border? Canada? They're Americans! Mexico? They just send over tons of illegals! (Note: I personally don't think those last two things.)
The rest of the world seems far away and not terribly relevant. And, to be honest, for most Americans that is probably fairly accurate. Unless a local factory is moved to South Korea, why would anyone be concerned about South Korea?
This contrasts with other countries which are constantly reminded of the rest of the world. Small countries - like say the Baltic States - in particular have very good reasons for wondering if their big neighbors will plow through them. Even major European countries - Germany, Poland, France - are constantly reminded of their neighbors on TV, radio etc.
This sometimes leads to silly reactions in the US like "France is a traitor." Well, even if you think French policies hurt the US, France is still a western democracy and in NATO (more or less). Sometimes it seems that people are equating France with the Soviet Union. Get some perspective people!
Back to the media point, American TV shows and movies are major exports around the world. Everyone gets a chance to see us (to the extent that Desperate Housewives and Charlie's Angels represent us). But how often do foreign shows end up in the US? Sure the British will show up on PBS, but more likely foreign shows are Americanized - from Big Brother to the Office. They even tried an American version of Faulty Towers once! (Staring Bea Arthur, forget the name of the show, it stunk and was quickly canceled.)
On top of this is the American ideal that we are the beacon of democracy in the world. In many respects this has been a good thing (Marshall Plan) but in other more recent foreign policy adventures - which shall remain nameless - it hasn't worked out so well. I think this gives us a superiority complex. And the worst part about it is, we have a pretty darn good country over all (there are major issues) and so that opinion isn't ridicilous.
So, basically I think it is because we are a big country, relatively isolated by geography, we have a superiority complex, and that we export culture, so most Americans don't need to know - and so don't care too much - about the rest of the world.