The NT has high population turnover (churn). People simply do not stay in the long term. Besides where would they stay? There are only really four actual towns/cities in the NT. A it limited, so the place is in reality Darwin centric. Darwin in this context includes Palmerston and the rural area.
A recent ABC article
People do not stay for a range of reasons. If you are working in remote communities you cannot stay there forever and unless the employee transfers to an urban NT location they usually head off for other places beyond the NT.
Then there are the cost and distance factors. Things cost more and people do not like being away from family so they move.
When you add in some poor working conditions especially from the NT Government in remote locations there is no surprise when people leave.