Or maybe he believes that damn pesky constitution (sarcasm) applies to everyone including the government when it comes to search and seizure, and to himself when it comes to right to due process of law, and the protection against self incrimination to name just a few.
Or maybe there is child pornography on there and he does not want to charged and tried and vilified in the court of public opinion. Right now he can stand on "higher ground" by making a constitutional law argument than exposing a secret that he may be ashamed of (guessing here).
Or maybe he truly does not know the password? Despite the governments best efforts they have not been able to crack it so it must be quite secure/complicated.