The US produce some excellent beers and sadly so few of them seem to make it to the UK. When they do get imported, they don't arrive with the economy of scale needed to compete on price with domesticity produced ales. Some supermarkets do import a few of the bigger names, but I'm sure the supermarket buyers just call up the distributors and say things like "Hey, we've heard great things about (insert name of US micro/craft brewery), we'd like to order their blandest beer." I mean do we really need another imported IPA on the supermarket shelves?
I have quite a list of beers I'd like to sample when next in the USA. So in order I pace myself and survive the next trip over there, I'll probably have to stay for a couple of months.