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.
I'm not going to argue that the USA is a great target for its relative monoculture, but relative is all it is. That the many cultures here are often a bit closer to one another than in most other places on the globe overlooks the fact that many of them are from vastly different backgrounds than the disparate cultures you'll find, for example, between nations in Europe and nations in Africa.
It also overlooks the fact that, to a similar extend that a language spoken in different parts of even a single country in Europe or Asia (China being a very good example) can differ to the point of unintelligibility from one to the other, a native Texan and a native of Louisiana, for example, can potentially speak the same language and totally fail to understand one another.
Like I said... yes, this is a great target. But this is not the total monoculture the rest of the world loves to call it.
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.