It is very much a question of how selling it to the fundors and the boxes they would like to have ticked (quite often they do not correspond to what the vision plan is, a major lacunae in government and administrative funding mechanisms often brought on by their need - ego does come into it too - to score in 2-3 years without having achieved the long term aims and vision and then somehow halfway through the whole thing goes to ground).
I could very easily make the case why in the masterplan A) elephants and B) expanding your savannah in terms of attractions and species are far more important for the long term vision Werribee is to project than out of place American bison will ever be! Admittedly and personally, I find that in the Australia environment North American and/or Eurasian species are actually a non-starter for exhibits simply habitat-ecotype-biotope wise.
Further, in the country at large you already have considerable issues with deliberately and accidental introduced exotics that are damaging the local ecosystem starting with "mega" vertebrate (dromedary, buffalo, red-fallow-sambar-hog-rusa-chital-deer) down to the lesser, but eventually some of the most damaging (brown hare, black rat, house mouse ..., red fox, cane toad, house sparrow, common starling, common carp, rainbow trout, mosquito fish ...)! An endless list, I am afraid.
Now, I can imagine that you would as a serious educational facility open up an exhibit on introduced species and use it to show how they are affecting the Australian environment (just fine). However, somehow species that are not typically found in an arid, semi-arid nor tropical north that which is Australia seems quite out of place here. It would be better if the local zoos and association take advantage of the environment that is there and how this could be a benefit for exhibiting particular kinds of species from similar habitat regions in Asia/Africa.
BTW: I meant all this in the atmosphere - location of the open range zoo concepts only.