The interesting part about the USA is, that the southern accent is closest to the original english spoken by the settlers. British english and other dialects evolved from somewhat that origin.

The differences between Australia and USA are also historic. Australia was used to imprison british people, hence the dialect evolved from there, but slowly due to low population.

In the US, the settlers were from all over europe helping the language to evolve in several directions.

Hungary for example, has also little to no dialects at all