They dont stop the zoos. If a zoo wants an animal they are allowed to get them. Problem is once breeding has begun if theres no other zoos that want the animals the individual zoo is stuck with them. There is only so much room in zoos for so many species to maintain healthy self sustaining populations so the AZA and its TAG groups try to carefully manage those spaces to maximize diversity, promote conservation and keep the population at good levels. Unless enough zoos want the species the AZA doesnt want space spent on a species there is little to no interest in or ability to build the population when it could be better used on something else. Unfortunately that means the TAGs have to make some tough calls and recommend phasing out some species in favour of others.
Maras might have been phased out once interest in them diminished. Thats not to say they wont rise again but there spaces might have gone to another species that is far more in need. And it could be that with a healthy population in Europe the AZA simply decided to refocus on other species. Other reason could have been at play. I for one liked the maras and miss them. They are like a cross between a bunny and a capybara and who cant love that.
