The US was the most prosperous country in the world in the mid 20th century. It was also the free'est market in the world at the time.
The little "thing" or status of having reserve currency has allowed the US to enjoy lower international borrowing costs than anywhere else in the world. This has immensely benefited the US, in facilitating ease of borrowing with relatively low interest rates and by facilitating lower exchange rate risk in borrowing than any other nation.