I feel like it's fair to assume the documents should contain something fairly scandalous if they're real. Is there another reason they wouldn't have been released earlier given the public interest?
The boring reason would be that the withheld disclosures contribute nothing new at all to public understanding and simply obscure irrelevant tangential details like adjacent programs, individual names and addresses for non-key figures, etc
There's a theory that one of the secret service agents accidentally discharged their rifle in a panic, killing JFK. The continued cover was to avoid embarrassment, but also avoid delusional maniacs from murdering them or their family.