A big part of it is obviously the relative effects of twentieth century wars, both hot and cold. However, we also need to be aware that in recent decades, the US has just offered a better deal for entrepreneurs than Europe. By that I mean it has been a democratic, rule-of-law-based country with relatively easy access to capital and relatively low taxes.
It still has the last two things, of course. But the first two are also essential, long-term, and if they're eroded then the US might stop looking like a better deal.