RFDS aircraft are always in Darwin. RDH is the closest major hospital to the top of WA, it also houses the national critical care and trauma response unit so the level of medical expertise is probably above the average bear hospital of that size. From KNX it would definitely make more sense to fly the patient to Darwin then to try and get them to Perth. Even if the jet was on the deck in Kununurra. On the jet its 50mins to Darwin v 3.30+ to Perth
It might just be prudent to have a facility where you end up flying to a bit more often than maybe was planned.