Things are (1) generally withheld for very boring reasons such as individual privacy [1], (2) considered boring and inconsequential with no reason to override the first concern and (3) there is a culture of overclassification.

Adding it all up a documentation release like this is awaited breathlessly by some people but can be summed up as "move along folks, nothing to see here" and will leave the breathless waiters still thinking there's a coverup but actually most of what the FBI does is a terrible bore, particularly when somebody like J. Edgar Hoover is in charge.

[1] sometimes non-trivial though, they might bring in an undercover agent who is peripherally involved, interview them, find nothing they don't already know, but it is possible this person could be the subject of retribution decades later if their undercover role is known