It's part of the complicated dance that prosecution has to do to not overstep the bounds of the 5th Amendment's ban on compelled testimony. 5th Amendment protec

It's part of the complicated dance that prosecution has to do to not overstep the bounds of the 5th Amendment's ban on compelled testimony. 5th Amendment protections means that if you compel someone to give you information, then that information and its immediate consequences are no longer court-admissible evidence. If there is any information that you don't want to "poison" with self-incrimination, you want to make sure that you can establish that information separately. So, if the court compelled the defendant to decrypt his hard drive, then the fact that that type of content was on his drives is a direct consequence, and therefore not court-admissible. However, now the existence of that category of content on drives under his control has been established independently. Therefore, the "cost" (in terms of information rendered inadmissible) of compelling decryption has been reduced considerably. (IANAL)