Among the countries colonized by England, many if not most of them speak english with a British dialect. Why is it that the US developed its own unique pronunciation, while a place like Australia kept it's British accent?
Obviously I understand there are differences in English dialect and Australian etc, but that difference is almost minuscule in comparison to American dialect vs English.
Do you really think Auzies sound like Brits? shrug I don't, but whatever. Canadians certainly do not.
However you should know that none of your ancestors two hundred and fifty years ago sounded like you do now, and they didn't sound like Americans either; language changes.
Just to look at the popular shibboleth in England today, Samuel Johnson said, "...'r' is never mute unless the second 'r' may be accounted mute." Yes, Dr. Johnson made that pronouncement. Here, check it. Now, to be fair, if you look into his notes, the 'r' he had in his upper class London dialect was not at all like the 'r' common in modern American, and to be clear there were already a lot of non-rhotic lower class people in London and swarms of them in all classes out in the midlands in the good doctor's time, but he was sure that his was the only right and proper way to speak (amazingly there are still people who think like that almost three hundred years later).
And non-rhoticity was not even the predominant dialect in England just seventy years ago (a sneeze in the evolution of language). Here is a map of roticity in 1950 and here's one for 2000. How this happened so quickly is a bit of a question in linguistics; most think it must be more than just the BBC's insistence on hiring presenters with public school accents because language keeps on changing in ways contrary to mass media speech (go ask r/linguistics).
Another possibly major factor is that many of the early English settlers in North America came from the channel ports and along the Irish Sea Coast, places with rhotic accents then, and the biggest 'r' preserves in England today. The convicts sent to Australia probably came from the city a hundred or so years later, and the non-convicts followed from the midlands a bit after.
BTW there are several American dialects that have been non-rhotic for longer than most of England. The most well known ones are the Boston Brahman accent, the Boston Southie accent, some from NYC and some from the coastal southeast. Just because I like it, here's a famous speech by a non-rhotic American
