What I don't get is there are less Francois' Langur in Australia than Dusky Langurs and Adelaide is maintaining their Langurs well, all we need is another pair and they would be alright for a while, but always more is better. I think Taronga and Melbourne should have Francois' Langur while Adelaide (and maybe another zoo, possibly Perth) have Dusky Langurs. Adelaide still has space to have more.
I suspect what the TAG would say is that there are only enough spaces for Langurs to maintain one species, and the preferred species, probably for conservation reasons, is the Francois' Langur.