At one time there were several types of synthetic (manmade) materials on the market for ACL repair. But the synthetic grafts weren't like the normal ligament. They didn't have the ability to repair or remodel. Other (human) graft material works much better.
When using a synthetic graft, the graft tension isn't very "forgiving." For example too much tension could result in a stiff joint. Not enough tension places the joint at risk of instability. Human tissue has a certain amount of "give" to it, allowing the graft to loosen up a little if it's too tight. This is called viscoelasticity. The synthetic graft has no viscoelastic properties.
On the plus side, synthetic grafts were free from problems at the harvest site. There was no disease transmission as can occur with tissue from a donor bank. Synthetic grafts were available off the shelf anytime a patient needed an ACL repair. Synthetic grafts can't repair themselves like natural ligaments can.