0) Banning of non-competes in Silicon Valley (!!) and ease of incorporation / listing on markets back in the 90's and before
1) Foreign investment in the U.S. (Saudi, Chinese, Russian money due to favorable treatment, anonymity (!)) and presence of U.S. dollar worldwide as a reserve currency - lots of that money ended up in Silicon Valley (Softbank etc). This especially matters since a lot of the compensation is in RSUs and the U.S. stocks have benefitted from above.
2) DotCom bubble because of (1) inflated salary expectations; U.S. had a head start over other countries also because of the DotCom bubble.
3) Personal bankruptcy laws are more lax than Europe I believe (although I'm not positive), since you can discharge all personal debt in a bankruptcy, cost of risk is lower (leads to more risk taking).
4) Gap between EU/US salaries has varied depending on exchange rate. Generally the way the world economy is set up tends to strengthen the dollar (everyone needs them to transact oil and pay foreign obligations after 1971/1974). That might be changing though; with a weaker dollar salary gap will naturally shrink.
5) As people mentioned before: vacation, school, healthcare cost differentials (though my hunch is that this is a small effect if any)
6) U.S. is a unified market of ~300 million people, most speaking English. EU is many smaller distinct markets because of language & cultural reasons (more-so than U.S. states), so more effort required for growth.
Probably some other reasons that slipped my mind at the moment.