This is because the surgical approach to the damaged ligament is dangerously close to blood vessels and nerves and the techniques are demanding.
If a PCL has torn itself from the bone, it can be re-attached if diagnosed early enough. A tear through the middle of the ligament can also often be repaired if found early enough, as the blood supply in this region is good.
Many PCL tears, however, are only diagnosed late, for a number of reasons, and then it is necessary to do a ligament replacement procedure. Like the ACL the procedure most frequently done in good units these days is a hamstrings graft using hamstrings tendons taken from the patient on the same side as the injury ('autograft').