Pileated is really rare- there's only the GPZ and the gibbon conservation center in California, and maybe one other places. Müeller's are phase out. I don't know how many of them are around anymore. The AZA wants pileated gibbons to be phased in.
There are lots of different reasons for phase ins/outs... Each species is different, though. I don't know why specifically for these gibbons. A lot of times it's due to a small founding population. Other times, the species has weird issues (like giant pittas). Perhaps the species isn't as attractor to the public any more (the reason why almost all of our macaques are gone). The last main reason could be that Europe has a strong plan. For example, we have a strong Arabian oryx program, so we phased out Beisa oryx and let Europe handle them.
AZA doesn't post most of the ins/outs online. The most reliable way would be to see if a certain TAG has a website (for example, snake tag, bear tag, antelope tag, and several others are). You have to be an AZA member to access them all, though. I've been using this thread, as it has a comprehensive (though slightly outdated) collection of them all: http://www.zoochat.com/22/phase-out-species-united-states-309373/
