There appear to be a number of reasons for phase outs. One is the size, average age and or relatedness of the population. Often these just not enough gentic diversity to maintain a healthy population. Of the tags that offered population numbers the pase outs usually had a population the was very small in comparison to the others in their tag. Another factor is just interest in keeping them. If zoos dont want them the wont be exhibit space for a healthy population. They might as well focus on species the zoos want. Third theres the ability of the species to reproduce. If too many are post reproductive that wont do. If the just dont like breeding in captivity they might as well keep others who do. We just cant take wild animals the way we used to. Fourth is the abilty to obtain new founders for a smaller gene pool. If you cant get enough founders and have little hope of getting them you are stuck. Fourth is the health of zoo populations areound the world. If the europeans have great population of a species it frees aza zoos up to breed other species that europe doesnt have. It doesnt appear they care about any other regions populations. Fifth is the situation of the animal in the wild. Theres not as much need to take up space with animals in the least concern category when they could breed rarer animals but not so rare they cant build a healthy reserve population. The goal is conservation balanced with a healthy genetically sound population for the next 100 years. Sith is the consideration of where is conservation money best spent. Will they do better in zoos or working on conserving the animals in the wild. Its a complex formula each tag must sort out for each species it manages. If you can think of a reason to keep a species around or one to get rid of them odds are theyve considered it.