War, duh. No, really, the only reason for that happening was a total war. War caused devastated countries to collectively sign Bretton-Woods which affirmed USD as a reference currency and allowed USA to externalize a lot of it's issue, both immediate and future. Allowing this externalization, plus major political influence in the first decades after the war, plus rapid innovation accelerated by the war allowed USA to become filthy rich, which allowed Homer Simpson to afford a mansion, car and 4 jobless dependents on a single government job.

Unfortunately the rapid global development means that even new world war wouldn't replicate this period. Train has left, bye bye, and won't return in our lifetimes. We need to adapt.