The U.S. is more wealthy and has a large private aviation community because private individuals can afford to buy and are permitted to fly their own small planes. As s result, it is easier for Americans to build up experience as aviators before going to work for an airline.
The other thing the US has is the 2 largest airforces in the world (The US Air Force, and the US Navy). This provides the opportunity for pilots to get a lot of flight time in jets without having to pay for it via flights schools or private aviation.
