No history reasons?
Some good answers (isolation, natural resources, etc).
But big reasons are the two world wars, especially the second one.
The U.S. came out of WWII as, well, the sh1t. We lots some folks, but not nearly as many people as the Europeans. Europe was destroyed and shaken. Factories destroyed, countryside ruined. Our factories were working at full production, many new, and none destroyed. European nations owed us money. We had the only really strong military left, so it was on us to tangle the only other remaining power, the USSR.
Lots of people, fully industrialized, incredibly large middle class, strong military, and a power vacuum after most of Europe caught their breath for half a century.
Since then? The other big military power died (USSR), leaving only the US. More than that, American culture has seemingly infiltrated every corner of the industrialized world... and if anyone has played Civ3 (I haven't), then you'd know you can win with "culture". The Germans love David Hasselhoff. Jeez.
Will it last forever? History tell us it won't... after all, how are Babylonia, Persia, Macedonia, Egypt, Rome, Turkey, Mongolia, Saudi Arabia, Portugal, Spain, France, Britain, and Germany doing now? Some better than others, but at one time at the top of the world. Maybe the EU will end up kicking America's butt in 50 years. We'll see. Maybe its started... I mean, I liked Run Lola Run and Brotherhood of the Wolf...