I would say its part genetics, part training, part chance.

Some athletes have for example bones that can handle a beating without breaking, some dont. Another part is training and how they have taken care of their body through the years. Some athletes push through the pain of an injury until they break, after this it can be hard to come back. There are also those that keep training when prehaps a rest day or two would benefit them. Huge amount of physical stress on the body will lead The last part is chance. Some are just lucky and some arent. It can for example be a tackle in some sport that if it were a millimeter higher, it could end a career. Nothing athletes really cant control.