grumpyoldgeek: ...but there's several reasons why you would not want to encrypt the data and not a single reason why you would....
Grumpy: In military aircraft there may be some desire to keep flight and voice data encrypted. The aircraft might be lost over hostile airspace and the adversary may get to the boxes first, and that data could prove useful to an adversary.
Here is recent press release for one such system:
Curtiss-Wright Announces Encrypted Cockpit Voice and Flight Data Recorder