If you go to this blog site I think you'll find a very interesting article about why the US is so far ahead of other countries in terms of GDP, employment, wealth and productivity. Many people that commented on this site have different opinions on why we're the richest, but the article I saw and liked said that we're richer for mainly three reasons.
"1. Institutional structures: political, administrative, legal-and-regulatory, financial, corporate governance, and educational;
2. Policy differences, especially in the degree of openness to foreign trade and investment, regulations that further or block market competition at home, vividly different levels of taxation and welfare transfers, and social expenditures in general;
3. Cultural attitudes toward two related things: 1) toward risk-taking and entrepreneurship; and 2) toward radical economic and social change that, if adapted to quickly, allows the Schumpeterian forces of creative destruction to operate freely, for all the dislocations such change entails to the status quo. "
To me this seems like the most logical argument toward the reasons about why the US has been the richest country since 1880.