Because the floor of caves is all covered in bat, er, "guano." They also hang from the walls, by the way.
That's what they would be asking about the rest of the creatures.
But they don't have strong legs or leg bones to support their weights.
They Might Not Know They Are Sleeping UpSide-Down, They're Bind Arent They. They Wouldnt Know.... :)
A bat hangs upside down from a secluded spot because it puts them in an ideal position for takeoff. Unlike birds, bats can't launch themselves into the air from the ground. Their wings don't produce enough lift to take off from a dead stop, and their hind legs are so small and underdeveloped that they can't run to build up the necessary takeoff speed. Instead, they use their front claws to climb to a high spot, and then fall into flight. By sleeping upside down in a high location, they are all set to launch if they need to escape.
Hanging upside down is also a great way to hide from danger. During the hours when most predators are active bats congregate where few animals would think to look and most can't reach. Effectively, this allows them to disappear from the world until night comes again.
To hang upside down, a bat flies into position, opens its claws and finds a surface to grip. To get the talons to grab hold of the surface, the bat simply lets its body relax. The weight of the upper body pulls down on the tendons connected to the talons, causing them to clench. Since it is gravity that keeps the talons closed, instead of a contracted muscle, the bat doesn't have to exert any energy to hang upside down. ♥