I think it is because Darwin is very small, population of around 122,000.
Yes, one may argue that international flights can be independ6of a cities population as the flights could be full with intefnatiinal tourists and then depart later with fhe same international tourists. This possibly works when the international destinations have a high wealth and a lot of wealthy internatiinal travellers. There also needs to be sufficient attractions to keep these wealthy travellers busy for a number of days/week or more.
Unfortunately, our Asian neighbours are n ot wealthy nor have a high proporation that travel (especially to remote locatoons like Darwin). If Darwin had a population like Sydney, Melbourne or Brisbane, then the number of flights woukd be considerably more with Australians leaving for an overseas destination.
Maybe the only way for international flight nunbers to increase would be for flights ex eastern capitals having a stopover in Darwin on route to the international destination. Some however may find this inconvenient and an unnecessary delay on their travels.