Respectfully, its mostly because anything that is successful in Europe is immediately purchased by the big US tech giants.
Mojang and Skype come to mind. As well as three of my former companies that are now owned by Oracle.
The EU market is definitely harder, it is not as contiguous and large as the US one, even the smallest required task: localisation, is hugely expensive.
So you can hit a critical scale faster in the US, sure, but any time anyone actually conquers Europe all that awaits them is acquisition.