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.