How did Americans develop an accent so far removed from the UK accent, even though Australia still sounds somewhat similar?

Languages and accents are forever in a state of constant change, affected by any of a dozen things in the environment the language and accent exist in. More can be said on the matter if anyone would like to provide their input; for the meantime, OP, as the question of how the American accent developed is a very common one on the subreddit, here are some previous posts for your perusal:

u/lord_mayor_of_reddit has an overview here;
u/keyilan notes some important facets of accents
and most recently, u/Harsimaja observes some changes in the accents.