You pose a very good question that hasn't been fully answered yet. It's easy to think patient motivation may be a key factor. But a new study may shed some light on this problem.
It's true that recovery from a complete tear of the ACL can occur without surgery but
with a good rehab effort. It's likely this is possible when no other injury has occurred in the knee. But when ligaments, tendons, cartilage, or capsule are torn in the back
corner of the joint, the joint loses its stability.
A very important structure is one of the hamstring tendons in this area. It works to hold the joint steady. Without this action, a torn ACL can't cope and becomes even more unstable. Researchers think it's possible this is the same reason why some repaired ACLs fail. If the inside back corner of the knee isn't fully functioning, the knee is unstable.
Sometimes repairing just the ACL isn't enough. Doctors must look carefully for any other damage, especially in the posteromedial corner.