I wonder what media they were using to store the information. If there are a lot of computer files for this then the question I have is did they consider constantly upgrading the storage media? NASA has lost 20% of the Viking Lander data, I read recently, and I suspect it is simply a case of either the media going bad or not having the ability to decode the format.
It would be unlikely the data on Fogbank was not maintained. It may have been mislabled but that would be by accident and not design. I imagine there is the possibility of espionage having happened in this case.