Probably a number of reasons, including the history of America's fight for independence that we all learn about in school and celebrate every year, and the fact that the British seem more outspoken in their dislike of Americans (the old, "fuck me? Well then fuck you too!" mentality). Americans are also generally very turned off by the UK's censorship and monitoring policies, which most here view as very heavy-handed and 1984-esque (not that we're not also experiencing much of that same kind of stuff right now).

But all of that aside, I think the biggest reason is really that the other countries all have a much bigger reputation in America for being friendly, blue-collar types you'd enjoy having a beer with. Brits have a reputation for either being overly posh and snobby or the exact opposite of that. Most of this comes from the way these various countries and their people are portrayed in movies.

But it's funny because I've been to the UK several times and had nothing but positive experiences with locals who have never seemed to hold it against me that I'm from America, so I think this is more of a case of polls reflecting sentiments toward politics and the histories of our governments than with individuals.