I’m guessing it’s because they are operating off testimony of a witness (defendants sister) claiming she was shown the alleged images, and since they weren’t on the unencrypted internal drive they MUST be on the encrypted external drives. That combined with the knowledge that these files were in fact purportedly known to be downloaded via his internet connection is enough for something, but all they have without the drives is hearsay, hence the compulsion to decrypt then?
Personally I think in this instance with recent rulings that an individual cannot be identified by an IP address and a single witness that there isn’t enough to KNOW anything, otherwise I could wardrive around, download a bunch of CP on somebodies connection and say I saw them looking at it through a window or something.
All I know is that we do have precedent for this in the physical world, so it’s not a logical leap to require disclosure of cryptographic keys when we KNOW what they unlock.