Immigration.

The USA is a country where you can have a new life and as a result it has drawn a lot of people from all over the world to come there.

Some of these immigrants and children of immigrants were very smart people such as Herman Hollerith (German heritage), Albert Einstein (Germany), Alexander Graham Bell (Scotland), Henry Ford (Father from Ireland), Walt Disney (ancestors from Ireland), etc.

These people have helped to create businesses and corporations that are now world leaders thanks to innovativeness and two world wars.

The USA was safe from the wars that raged through Europe so it's businesses got a competitive advantage.

The USA has a great media empire so their values and ideas are spread across the world faster than any other nation.

The USAs culture is very impressive because of it's melting together of different nationalities. Jazz and R & B (African and American influences), Andy Worhol (Slovakian ancestors), etc.

I suppose you could say that the USA is great because of every other nationality in the world.