I would imagine the case against him for murder or wilful murder wasn't strong enough for them to risk the prosecution failing. There must have been a lot of circumstantial evidence. They needed the body and although he confessed to a plea of manslaughter the fact that he didn't then reveal it's location screams that it was far worse even than he said it was. Based on the assumption that he has and still is concealing evidence to prevent his full crimes from being revealed how can they possibly claim he's been rehabilitated? He's still in denial and hiding the truth.
