Europe (mostly, at least) does have a unified market on paper, but not culturally, which makes marketing, localization etc. harder - a commercial that sells well to Germans might go down like a lead balloon in Spain.

The biggest advantage I've seen in the US has been the VC ecosystem. For all we deride VCs - and sometimes (particularly given events over the past few weeks) with very good reason, the money spigot is just much more generous in the US. Of course that will change in the new low interest rate economy, but that to me seems more the deciding factor than silly claims that Americans are somehow more innately "innovative" than Europeans.