The monkeys at the clinic are the cutest and usually most receptive to volunteers.
Although quite intelligent these monkeys have little to no instinct. Instinct, is usually the main factor determining whether an animal can be released into the wilderness. When these monkeys arrive at a clinic like Proyecto Asis they grow accustomed to human behavior and cannot be placed in the wilderness again because they’d likely treat a predator the way they do a human.
These monkeys learn the ways of a wild spider monkey from their mother so when they are hunted and their mothers killed they can no longer learn how to survive in the wild.