It is explained in the ruling, explicitly.
Page 8: "But the following question remains: Is it reasonably clear, in the absence of compelled decryption, that Feldman actually has access to and control over the encrypted storage devices and, therefore, the files contained therein?"
Page 9: "I conclude that Feldman’s act of production, which would necessarily require his using a password of some type to decrypt the storage device, would be tantamount to telling the government something it does not already know with “reasonably particularity”—namely, that Feldman has personal access to and control over the encrypted storage devices."
Paraphrased:
> If the government has reasonable suspicion that illicit media exists, and can verify you have access to that illicit media, it can compel you to retrieve that media.
However,
> If the government has reasonable suspicion that illicit media exists, but can not verify you have access to that illicit media, it can compel you to retrieve that media.
Even if the government could access the media, it would still have to demonstrate that the defendant could also access the media. By accessing the illicit media, the defendent is demonstrating possession of the illicit media. This amounts to self-incrimination, which is protected by the Fifth amendment.