One obvious reason is that these documents can confirm spying operations that were happening and may still be continuing to today. For example the US was tappi

One obvious reason is that these documents can confirm spying operations that were happening and may still be continuing to today. For example the US was tapping and transcribing all the phone calls to communist bloc embassies in Mexico in collaboration with Mexico’s presidency but unbeknownst to their law enforcement agencies. Of course we already safely assumed they were tapping communist embassies, but the documents reveal the precise nature of the taps, what they usually did with the information they found, and to what extent the Mexican government was aware. There must be a lot of review and consideration before deciding to reveal these details, even decades later.