Australians are descendants of British criminals. Their dialect is supposed to sound similar.
A dialect changes because of mainly two reasons,
1. Isolation from the original language
2. Contact with other languages
I have a suugo-science theory that dialects evolved faster before written language, in more primitive civilizations. Everything is documented today, and people are generally more educated which leads to less evolvement, or derailment on the languages/dialects. We live in a globalized world, so isolation is not a factor anymore.
The first British Americans were isolated from the "England English" speakers and came in contact with Native Americans. Also other settlers from Sweden, Spain, France, and the Netherlands.