1. Mechanism/strength of the ligament: The ACL is thinner and biomechanically weaker than the PCL, and it acts as the primary restraint against anterior tibial translation and internal rotation of the tibia on the femur. It is therefore easily strained by the common noncontact mechanisms of sport - sudden deceleration, cutting/pivoting, hyperextension, or excessive internal rotation of a planted leg - Gray's Anatomy for Students, and Tintinalli's Emergency Medicine (ANTERIOR CRUCIATE LIGAMENT INJURIES).

2. Force required and protective anatomy: The PCL is thicker, stronger, and more vertically oriented, and it is additionally protected by the posterior capsule, hamstrings, and popliteus. Injuring it requires a much greater, usually direct force (e.g., a blow to the front of a flexed tibia such as a dashboard injury, or violent hyperextension), so isolated PCL tears are rare - Gray's Anatomy for Students ("Cruciate ligament injuries").

In short: the ACL is subjected to everyday rotational/deceleration stresses in sport while being the weaker of the two ligaments, whereas the PCL is stronger and needs high-energy trauma to fail - making ACL injury far more common.