The standard of living in much of the EU is pretty bad, and in the rest there generally aren't many people. There may be 800 million total, but only a small subset of them are marketable.

The U.S. has almost as much GDP as the entire EU, while having <40% of the population which, most importantly, means a lot more discretionary income.

I hope you didn't take my statements as jingoistic, it's just a well-known fact that U.S. users monetize much better than the rest of the world in general.