The reason is that ligaments are poorly vascularized. In other words, there aren't many blood vessels to provide nutrients for the ligaments. And without nutrients tissue repair is not possible. Oftentimes, ACL tears require a surgical graft.
For parents, it's axiomatic that steering young children into sports is good for their overall health. But just like almost everything else, too much of a good thing can also backfire. Playing a sport too rigorously can produce devastating injuries for teens who are still developing, as can playing one sport exclusively year-round.