My Husband Sustained A Blow To The Head And Was

My husband sustained a blow to the head and was knocked unconscience for shirt period of time and diagnosed with severe concussion. He had just about all of the signs of a TBI. He has dealt with this for 7 months now. He is getting help now for speech and OT. He is still having trouble with memory loss. He has lots of medical issues and on lots of medication. My question is why is it taking so long for him to regain his memory?