The cruciate ligaments, especially the anterior one (ACL), are put under enormous strain. They keep the knee from moving in the wrong direction, and there's a lot of energy in a moving body. If something causes the foot to stick in the ground while the rest of the body moves in a different direction, it's easy to tear that ligament.

If the ligament isn't completely torn, it can heal, but it must heal completely. Otherwise, it will be quickly torn again.

Often, the ligament is completely torn, and it must be replaced, usually with a bit of your own tendon from a different part of the knee. That must be screwed into place, and then it must heal completely to be strong enough to bear the tension. With all that time off, the muscles have to be rebuilt.

Bones heal rather quickly, as do skin injuries. But the ligaments don't receive a lot of blood, and it takes time for them to re-build the very tough fibers that make up the ligaments. Any ligament injury takes a long time to heal; the ACL just happens to be torn frequently because the knee is a very vulnerable position.
Source(s):
Personal experience with a torn ACL

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