There's a number of factors related to that, I think. I'm engaging in informed speculation here, based on my experiences with my fellow citizens.
I think the short answer is that the US is physically huge. Just the state I live in alone is about the same land area as Germany, and there are 49 others -- many of which are larger than mine.
Just getting to an airport that you can take an international flight to the UK from can be a big deal all by itself. It's not cheap (an oddity about plane fares in the US is that domestic travel can cost as much or more than flying across the Atlantic), and can take a day or two. The cost of the flight from, say, New York to London is not necessarily the major portion of the travel expense.
The travel time is significant, as I mentioned. For a lot of people (pretty much anyone who doesn't live in the general north-eastern portion of the US), a "weekend trip" would require 4-6 days. Vacation time is limited, so not everyone is willing to spend a significant portion of it on an airplane or in airports.
Cost is a big deal -- traveling overseas is expensive, and the prices for things in the UK are quite high as well (probably on par with New York, but New York is very expensive). Most people who aren't wealthy (which is the vast majority of people) could really only expect to afford it once or twice in a lifetime, unless they are willing to sacrifice a lot of other things to make it happen more frequently -- and some people do, but most people have other priorities for the money.
(I wonder why your comments keep getting downvoted? That's weird.)