I've heard the same - essentially due to them being a high risk species they require a lot more 'licenses' to be able to hold them. I know of a few people who tried to acquire some as 'pets' but couldn't it's just too difficult considering all the regulations that are in place.Foxes are kept most people do not have the room, you need special licensing to keep them as they are a kill on sight feral type deal. If you want to keep them they have to be fully contained in an essentially a giant aviary type cage. Most people dont have that room. But i have met a few people over the years who have kept the odd fox. Usually finding an abandoned kit and raising it.
I imagine the same would be the case for major zoos and there'd be little interest in accommodating such species when they should essentially be focusing on other 'priority' species. Camels are quite easy to just throw in a mixed species paddock and advertise them as 'ambassador animals'. There would need to be much more investments in say foxes.
So whilst I do think it would be a pretty cool idea to have a 'Feral animal' themed precinct, however you can see why zoos have little interest in that when they can instead focus on natives/more appealing exotics.