Avalon is too far from Melbourne. It's closer to Geelong, which is about 100km SW of Melbourne.
Tullamarine is connected to Melbourne CBD by a very good freeway system (even if the silly tolls play havoc with those of us who drive from interstate!)
Bankstown is probably too small for the size of DJ's operation. I'm not sure about runway lengths, but I think it's barely enough for a 73G or 738 which is what they're operating. Bankstown also has a relatively poor transport system for the city and is in the relatively lower socio-economic areas of Sydney. Thus, it's further away from larger markets. Although some might argue that DJ's market is lower socio-economic groups, some families in this region cannot afford basic things like housing, let alone family holidays...cheaper for them to drive if they go on holiday. Also, because of the jet ops, flight plans, approach procedures and aircraft noise procedures etc have to be modified...too complex.
M88, 722, 732, 733, 734, 73G, 73H, 742, 743, 744, 752, 762, 763, 772, 773, 77W, 320, 332, 333, 345, 388, DH8, SF3 - want