It's about money and everything else too. US is a really nice country to live in for high skilled individuals. An immigrant nation who speaks the international language, a fast growing economy, especially in cutting edge tech, with overall taxes lower than Europe and even the housing market crisis is not at bad as what you will encounter in many European cities. And its huge so for a lot of emigrants looking for a new home, they will tend towards the country with the huge economy so that when they get their new passport it will be worth something.
The only downside of the US was always the immigration system.
From a purely financial perspective, a country like Denmark for example, would need to pay more than the US to be as attractive, to account for the fact that it is a tiny country where the main language is not English and where the overall career prospects are more limited.