Thereby hangs a tail
When moving around in the trees, the more appendages you can grip with, the better. Many New World monkeys have prehensile tails. Spider monkeys are the most acrobatic. They use their tail like it's a fifth limb. It is strong enough to support their entire body weight, leaving their hands free to pick fruit. We think of hands and dexterity as very important to primates, but in fact when travelling fast though the trees all you really need is a hook to grab on with - a thumb only gets in the way. Spider monkeys' thumbs are very much reduced.