Some English used in the US was deprecated on main, but it still was in use in the fork.
American English used a number of things that fell out of fashion in GB/UK, but the US kept them. It also doesn't help that some non-major GB dialects were over-represented in some early settlements
Honestly moving to american english would be a good thing for britain. I've seen even native mfs failing to understand each other's pronunciation. British english has something like 18 vowel sounds but only 6 vowel characters. Multiple letters have the same sound and a single letter can have multiple sounds. That's not what i call an alphabet. Even american english has lots of unnecessarily complexity, but it made a step in the right direction. Ideally, many more such steps should follow.
I think American English (simplified English - as a yank, I think that should be the official term) is close to an XFCE or *box spin. Things like the dropping of "u" in words like colour were done for printing efficiency.
Aluminum is one where I think Americans have it correct. The British chemist who isolated it wanted to call it alumium, but it didn’t fit with the Latin-derived naming schemes of elements used at the time. Someone else suggested “aluminium”, where as the guy who isolated it went with “Aluminum”. So I say we stick with aluminum, or go to alumium cuz it sounds cool.
Most differences in UK-US English are due to the US sticking with original pronunciations of words, with changes being made by the UK, OR by the US focusing more on Latin-derived pronunciations of worlds of Latin origin. It's literally "The US didn't change the language drastically, the UK just changed the words and pronunciations of things and decided that the ones who didn't change are wrong"
As I've pointed out before, the United States is home to more native English speakers than the next three countries combined; none of those countries is England. English is 5th. England doesn't get to dictate how English works anymore. It's America's language now, and the way we do it is the correct way.