Well, from reading the opinion it seems that one important factor was that basically the government's position was "we don't know if there's hidden encrypted data here, and if there is we don't know if that data is relevant to the case". So keep that in mind when interpreting this.
even further, the prosecution has not indicicated a specific file or location he expects to find, based on other evidence, on the drive.... and the court is deciding that amounts to a fishing expedition. they want to see his decrypted drive because it might contain evidence... not because they are very certain it had a key piece.
even more important, although brief, is that by decrypting the drive the defendant would be automatically admitting he had control over the drive and its contents... something otherwise arguable on an unenecrypted drive, meaning he woud be testifying against himself for any illegal material found, even. if unrelated to thecase.