How did the American and Australian accent develop when they both had the same ancestors?

I mean, they both came from England, and some of them settled in America while others in Australia.

Why did the Australian accent develop, and why did the American accent develop. Why couldn't they have kept talking with an English accent?

I'm trying to understand the development of an accent, which is an interesting topic to me.

Hope I'm making sense, english is not my first language