It just seems that once a bird dies or is taken off display for some reason it isn't replaced (eg. fireback pheasant) and there are other birds that don't have a mate (green peafowl).
Just to use this as an example, and only a quick reply I'm afraid, but hopefully this might help. I'm no aviculturalist, but I'd bet that fireback pheasants and green peafowl are probably available in the private bird market in Australia (I'm happy to be corrected if this is wrong). The region's zoos have
nine different pheasant species listed in their collection plans, and this is probably overkill, so deleting one or two of these is no hardship in the bigger scheme of things.
There are 24 green peafowl in the region's zoos (granted, 12 of these are in Fiji), so there is no shortage of them, if Taronga (presumably the zoo you are talking about that only has one) wants to obtain more. In fact, Taronga are phasing the species out of their collection, so are intentionally not obtaining any more.
But let's say, there were only a few fireback pheasants left in the country. Zoos are not going to go through the massive logistics of sourcing animals overseas, from countries where they can be imported, and then go through extremely costly import procedures and quarantine procedures for this species, especially when there are plenty of other pheasants around. And since only 3 zoos are showing any interest in the species, the small number of birds that zoos would be interested in importing, would make it totally cost-prohibitive.
The comment from the regional bird TAG for this species is:
"
Species potentially reproductively extinct within the region. Although a small number of specimens are known to exist in private hands and a smaller number in zoos there has been no reported breeding success for a number of years despite serious attempts in recent times. It appears that repeated inbreeding has resulted in the loss of fertility in this species. Institutions with spaces available for Lophura pheasants should allocate these to the regionally available Kalij, Swinhoe or Silver Pheasants. There are no import health standards in place for this species to facilitate import from outside of the region."
As Glyn mentioned, there are far higher priority non-avian species that
can be imported, with a lost less cost and hassle. And since Australia has one of the most exciting collections of wild bird species - we can still have wonderful displays of those.
I hope this helps.