I think the biggest problem in europe are the big differences in language and culture. The US has a population of 315 million people all speaking english and having roughly the same culture. In europe, the biggest language group is german with 95 million people.

A US startup hasn't a big problem to expand to Canada/Australia/UK which adds another 120 million people.

I'm working in a swiss startup and after expanding to Germany/Austria, we have to go to America because other countries (except probably france) just aren't worth the hassle (translations/PR/cultural changes like number formats). The problem now is that the jump over the Atlantic is not nearly as easy as adjusting your your google/facebook ads to show up in New York.