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.

There’s a host of factors, I’ll try to outline the ones I can think off off the top of my head

1. Industrial base: The UK was bombed during WWII, the US was not. That gave the US a production advantage.
2. Natural Resources: The UK was dependent upon many resources from their empire while the US was using a lot of their own domestic resources for production.
3. Decolonialization: the UK’s resource base, as mentioned in the last point, were largely seeking independence in the wake of the war. The US brand of imperialism was more economic in nature than political so they didn’t have the same issue.
4. Population: it’s tough to outproduce a nation that’s 3x the size (UK pop was about 50 million in 1950; the US was roughly 150 million)
5. The Marshall plan. It’s hard to overstate how much of a boon rebuilding Europe was for the American economy.
6. Debt: military goods aren’t cheap. The UK sourced a ton of war material from the US. I just looked it up; the UK made its final WW2 debt payment to the US in 2006. Sheesh!

not the UK who histrionically dominated most of it

I’m sure “histrionically” is a typo, but it still kind of works.

Because that was the deal for them to choose sides during WW2, now they’re at the end of that deal.

The UK just about lost and probably would have if the US didn’t get fully involved, and the big kid on the block need help and then the helper became the leader.

Which country is next, every empire collapses.

Because there were two world wars that mainly took place in Europe. America was never subjected to bombing or invasion, meaning their industrial capacity was never crippled. They came out on top each time and used their influence and strength to become the new superpower.

This is overly simplified of course.

my uneducated guess would be guns/military and computers.

They massively financially helped and “sustained democracies” that then welcomed USA’s products and culture, growing and becoming world powers themselves (like Italy). The B.E. had a similar chance, but way before, and they just did prey on everything they could (like India or Egypt) until they were kicked out.

because the UK gave up most of that power in the second half of the 20th century, now the UK is a relatively small country (by area and population) that is increasingly isolating itself from the world (Brexit) rather than attempting to influence global politics a lot more than other countries of comparable size, population, and wealth

But the US is slowly losing influence too. In 1990 its side had won the Cold War, but since then other things have happened: 9/11, Iraq War, George W. Bush, the war on terror, Donald Trump; many geopolitical events of the present can be explained (in part) by the fact that the US is losing influence over the world.

The US has a military base in every country, our boot is on the neck of the world.

Don’t worry tho soon the boot will be Xi’s and we will dissappear before we can even think about organizing

I am talking out my ass, but didn’t the UK basically overextend itself with its colonies and have to let them loose so it could focus on issues at home?