Sadly, an animal abandoning their baby is a natural part of the world. That doesn't mean that they have to die though. Humans can interfere and save an animal that otherwise would have been a goner. That's just what they did with this mountain lion cub.
Found on Indian reservation in Arizona, she was found by herself. She was sick and they brought her to a vet. The vet didn't think the cub was going to make it, she hadn't had any water or food in days. They were shocked that the she had even survived this long. Still, they were determined to do their best to save this little girl.
H/T: Arizona GameAndFish