Others has given good reasons, but I think it's hard to overestimate the effect of 'rich gets richer'.

Once you have the stronger pool of talent you get the better companies, you get more income, higher salaries and talented people wants to work with talented people. As a young geek I was very attracted to the tech environments in the US, MIT and silicon valley, and other places were not really on my mind. Even though there are competent places in Europe as well.

> Once you have the stronger pool of talent you get the better companies

Don't really have anything to back up what I'm about to say except a gut feeling. But Europe actually has got plenty of talent. European business just doesn't seem to value that or have ideas what to do with it.