CBR direct to brisbane conflicts with inbound traffic from south and west that are on descent to the main ingress point for Sydney, a place called Bindook in the Blue Mountains, they also conflict primarily with the main south route via Marulan and then conflict with the westbound/asian route that departs via katoomba.

By routing flights via sydney, lateral aircraft separation is possible because the flights are flying the same heading as those inbound to Sydney and can maintain distance deparation whilst climbing through conflicting altitudes, lateral separation is not possible if the flight tracks cross or intersect. Once over Sydney the flights track to west Maitland to join the main route to BNE, again they are on the same track as the conflicting traffic so once again the altitudes can be managed. This track takes any conflict with SYD-MEL/ADL or Syd/West/Asia departures out of the equation, in short a lot less work and risk for the air traffic controllers working the airspace.

I used to work Sydney Radar on the Sydney South sectors many years go and my post is based on this experience, the actual tracking points might be slighty different now owing to more use of great circle navigation as opposed to navaid routing but i am sure you get the picture!