The reason is brand positioning. You don't want pax to spend time with another carrier for ANY time. Each airline has a different strategy/rationale for when to develop its own lounge -- depends on the number of full-fare C/F pax and the number of flights. MEL would likely have a lounge for each carrier that has 2 or more daily flights with reasonable numbers of C/F pax. For CX, MEL market is a no-brainer for why to develop/operate its own lounge.