Because we came out on top at the end of WWII, but we were the main Allied nation whose country didn’t get blown to smithereens during the war due to being an ocean away. (Granted, neither was Australia but they were not and did not become a manufacturing powerhouse in the process.)

All of the European colonial powers lost a ton of their colonies either during or in the immediate aftermath of the second world war, especially the British empire. Australia is even included in that list, becoming independent in 1942. The rest looks like a who’s-who of former British colonies and protectorates, the most impactful and arguably the most famous being India in 1947. Also Jordan (1946), Myanmar/Burma (1948), Sri Lanka (1948), Israel (carved out of the British mandate of Palestine, also 1948), and many others in the intervening decades.

The Brits had to dedicate most of their military forces to fighting the war which left their various colonies undermanned. India’s independence in particular put into motion the expectation that all of these lands and protectorates could self-determine, and since Britain was A) broke, and B) imperialism was becoming progressively less socially acceptable in Europe, Britain let most of them go. Not least of which because they did not have the manpower to spend keeping those pesky natives down, nor did they have the money to spend paying anyone to do so for them.

America, meanwhile, built huge swathes of industrial capacity during the war which was all still there afterwards, owned significant amounts of debt from the various European powers from loans made and equipment provided before we entered the war fully, essentially owned Japan for a decade or two, and importantly did not suffer any damage to its own infrastructure, factories, or civilian populations due to being separated from both theaters of war by an entire ocean each.

TL;DR: Pretty much everyone involved in the war was left with a country made of rubble and ashes in varying degrees, except the US.