Max
History and slots!
The Q-link Dash 8 ops have grown out of many of the smaller early mover commuter airlines, and to some degree picked some of the old EastWest Routes.
Rex has evolved from Hazelton and Kendell Airlines. Both of which benefited enormously as and when Ansett exited regional routes. Hazelton also picked up some of EastWest's routes.
Along the way Q-link and Rex own some very good slots at Sydney, which for regional NSW have been effectively capped since 2001, and since VARA has come along they have picked up anything remaining worth having. So what you will see is that Q-link, Rex and VARA will use these slots for the routes with the best return.
As for South Australia there is probably only one route that could benefit from competition and I doubt it worth Q-link the cost to set-up a new base.