Nature Abhors a vacuum. When the treetops offered a niche, Some of the smaller primates made it their home and no doubt evolved specifically to it. The fact that larger primates and other predators had difficulty getting to the tree tops, would have made it much easier for a squirrel monkey, or a lemur to get away, they would have had difficulty defending them selves against anything, and they can't run very fast. A lot of squirrel monkeys who got too close to the ground, wouldn't have had any descendants. If everything could climb trees, there probably wouldn't have been any squirrel monkeys now.