@captain alligator Basically bad survival rates and lack of longevity, believed to be related to climate and diet reasons. At Apenheul specifically several individuals died prematurely, and they decided to send the last one back to Singapore and stop attempts to keep the species. Proboscis monkeys do seem to do relatively well in zoos in subtropical Asia.
@KevinB Incidentally, Proboscis monkeys are currently kept outside Southeast Asia only at Zoorasia in Japan and Chimelong Safari Park in China. (In any case, they are now only kept in zoos in Asia.)
