I think another major reason, is that the UK and europe in general doesn't have the look on drugs that the US (and maybe aussie) has. Drugs are MUCH more tolerated in europe, and generally more accepted. You have to realize that you never hear too often of UK officials busting down on boats smuggling heroin, maybe we just don't get the news all the way over here, but if the UK were to make major drug busts, I think the US media would show it just to make drugs look that much worse.
Also, I think price has a major issue to do with it. In the states, your average price is PROBABLY about $20-25 if you go to a club to get it. So for the price of one, people in england can get anywhere from 5-10 pills. Also you must realize, if your NEW to a drug, say extasy, you will most likely not be willing to spend more than $10-20 as it is. I know that if someone told me they could get a SUPER great drug and it only cost me $50 i would NOT buy it. I don't even know if its good!
I'm thinking that because of this, most americans try ONE pill their first time. As its still one of the best experiences they've felt, they get the notion that one pill is the appropriate amount and therefore don't feel the need to try more. Eventually as tolerances go up, they take more of course.
Also, since its so cheap and readily available in england, they don't have to worry with the side effects as harsh as we do. As phreax mentioned in his meth post, "we never really knew we were addicited since we never layed off the meth long enough to realize we needed it". Apply that to someone who can buy xtc pills for only $2-5 bucks a piece...
Just a few of my own theories, but I think bob has covered most everything there is to it!
Trips