Why is recovery time for an ACL injury so long?

Graft maturation. Besides regaining motion and strength after ACL reconstruction, your knee takes a long time to fully recover because the graft that is used to replace your torn ligament needs to incorporate into the bony sockets drilled to hold the graft. The graft also takes time to literally become a ligament. Cutting sports are usually not recommended until after the 4th to 6th month after surgery.
Can't rush nature. The surgery to reconstruct the ACL is about 2 hours, give or take. But the rehab is about 6 months, because that's how long it takes to regain your full motion, strength and functional ability.
ACl heals upto 9 mo. When you have ACL reconstruction, a new ligament is created and held in your knee with screws or other fixation devices. The ACL then has to heal in this position in order to withstand repetitive stresses associated with sports activities. The problem with any type of ACL graft fixation method is that, if repeatedly stressed before the graft has healed in position, the fixation will fail.