They are probably not considered for the same reason a whole lot of interesting species are not kept - not considered "box office."
I'm quite disillusioned with the apparent outlook of all the large Australian zoos today; while paying lip service to science/conservation/ providing a good exhibition of the natural world, their real major purpose seems to be financial. They are becoming more and more expensive to visit, and everything seems to be geared towards getting more money out of visitors once they are through the gates. I'm talking about zoos which are to a large degree government supported financially.
There are fewer and fewer species kept and if an animal is not "charismatic megafauna" then it is likely allowed to die out or, if we are lucky, one of the smaller private zoos will keep it going.
I'm an old guy. I can remember (in the 1960s) walking into Adelaide Zoo and being able to see 20 different species of monkeys or visiting Melbourne Zoo and seeing 6 species (9 races) of bears or going to Taronga in Sydney and being able to see 14 different species of Birds of Paradise. While no-one realistically expects to see these today, there is no real reason apart from financial for large Aussie zoos to get rid of animals like clouded leopards, Asiatic golden cats, De Brazza's guenons, jaguars, leopards, maned wolves etc.