British started shipping their worst criminals to Australia because America won the Revolution.
After 1780, the British could no longer use their penal colonies in Maryland, Georgia, and Virginia and had to find another place.
Sending criminals far away from home was an extreme punishment only reserved for the worst offenders, but it meant avoiding the costs of building new jails and feeding the prisoners.
So it didn’t take long for Australia to become Britain’s penal colony of choice.