They became the most powerful because of circumstance. After the second world war they alone were one of the largest countries unscathed by war. While others spend years rebuilding their infrastructure and economy the US was allowed to expand unabated. Sure they helped but that cost was small in compared to what most of western Europe, the Soviet's, and Asia were trying to recover from. Even their own Atomic and Rocket programs were highly indebted to European scientists that fled European tyranny  (Nazi's). Many of the Nuclear scientists were not American and their rocket program relied heavily on former German rocket scientists who fled the Soviets after the war. Their current education system is falling behind many countries now and the cost of their post high school education is becoming in-affordable to many.