Decrypting his hard drive may lead to other charges against other people. If he knows that he can incriminate others, he may be waiting for a deal.
They're playing hardball with him right now. After six months in jail, watch them offer him a deal if he unlocks the hard drive. He gets a firm but reduced sentence (including time served) and they get more bad guys.
(Why yes, I have watched too many episodes of Law & Order, why do you ask?)