The almost-superset relationship of the respective non-waivable human/employee/consumer rights, and the weak enforcement of a level playing ground in each market.

If you want to have competitive business both sides of the pond, the logical choice is to put the money on the American side and achieve ROI before the EU dishes out noteworthy fines for non-compliance. In maybe 10 years when you have outgrown those sums. Much cheaper/easier than the other way around: build a company in Europe and to European standards.. and then try to make it skip all the costly procedures it may legally skip when doing business with/against Americans.