Why do the two Australian territories have reduced levels of government?

There are currently three Australian territories, Christmas Island, Norfolk Island, and Cocos (Keeling) Islands.

Each has a reduced level of government when compared to the states/mainland because the local populations are so much smaller, typically less than 2,000 people but with one, the Cocos (Keeling) Islands, has less than 600 local residents.

There is however, debate around potentially giving the Norfolk Islands the same self-government status currently held by the ACT and NT. If this eventuates, then the population there of under 2,000 will still qualify.