The major reason for this is World War II. In that war, all of the other major powers in the world were left devastated. Their economies were severely weakened and they could not wield the kind of power that they had before. This meant that the US was, almost by default, the richest and most powerful country in the world.
In addition, the Cold War forced the US into a position as the leader of the free world. The US could not retreat into isolation as it had after WWI. Instead, it had to take the lead in opposing the Soviet Union and communist expansion.
The devastation of WWII and the needs of the Cold War made the US the most powerful country in the world and a global leader.
America has had two very properous times in the 20th century. One was the 1920's which unfortunately set us up for the Great Depression. In a fact, therefore, it was only a bubble. It was the era of the robber barons who took advantage of the poor to build great empires for themselves. Too much money at the top failed, and the economy shrank for a long period of time.
Second, the 1950's would be the winner because of its staying power. It started with WWII in part because of the way the war devastated our economic competion. However, I believe its main strength was people's willingness to pull together to get things done. The GI's returned, and we rewarded them with an education in the form of the GI Bill. The rich paid taxes at rates that would scare the pants off of the wealthy of our era.The top marginal tax rate in the US has ranged from a high of 94 percent during World War II to 91 percent from 1950 to 1963 then gradually falling to the current rate of 35 percent. It got the bills paid. People of that era knew there was no gain without pain, and everyone shared the pain especially those who had the most to give. We paid for our wars with patriotic fervor. We rewarded our warriors who worked hard to build the greatest economy our country has ever seen. They have been called the greatest generation for a reason.
We have learned nothing from these eras. We are victims of our ignorance. Those who do not remember history.......
Well also WWI put the USA on the map as an important Nation.