I would take some of those numbers with a grain of salt. US entries only started getting added a month ago, and there are still many holdings left to enter as well as errors to fix. As an example, I suspect there are more than 5 US zoos with Eastern Grey Kangaroo...
Also worth noting that some species being in low numbers aren't necessarily a sign that they are "disappearing". Tasmanian Devils in the US have always been dependent on imports from Australia, so they've never been common and their numbers fluctuate based on when the most recent imports were. Also not sure either of the wombats are actually in decline; Common Wombats are not a breeding species here, and Hairy-nosed I think have had more or less the same number of zoos holding them for a while.
Feathertail gliders are not disappearing. Yes, there are only a few holders, but they've never been common and some of those zoos have very large colonies.According to ZTL, the following marsupials are kept in up to 5 US zoos
The southern hairy-nosed wombat and eastern grey kangaroo are held at 5 US zoos.
Derby's woolly opossum, parma wallaby, swamp wallaby, yellow-footed rock wallaby and narrow-toed feather-tailed glider are held at 4 US zoos.
The Tasmanian devil is held at 3 US zoos.
The New Guinea ground cuscus, agile wallaby and common wombat are held at 2 US zoos.
The four-eyed opossum, bear cuscus and grey short-tailed opossum are held at 1 US zoo.