I don't know how things work with licencing today, but in the past, PD added the Ginetta, Spyker, that Dutch yellow race car and TVR as examples. Again, I don't know how long PD had the licence to use them. The XR8 stayed through to GT6(and GTPSP).
Might be different with defunct cars. Maybe Ford(USA) had the last say over Ford Australia or maybe there was too much red tape between Ford USA/Ford Australia/FPR/FPV/Prodrive/David Richards, to include the F6 & GT in GT6.
Ian Bell from SMS(Project Cars), mentioned they almost got the licence for the series, but VASC wanted too much to include all the tracks. Hope that's not the case of why there are no Australian cars(as yet) in GTS.