Probably because nobody has bothered to review them and decide whether or not they should still be classified. I talked to my FIL who was a test pilot about declassification and he said it’s mostly random. Stuff stays classified for many years after it’s common knowledge because nobody really bothers to check.
There are rules about reviewing and declassifying information. The rules get applied at years 10, 15 and I can’t remember the longer ones from the date the material was generated. In the absence of special waivers the material gets declassified. And you can bet those 28 pages do get reviewed rather than slipping through the cracks.
As an aside, it’s these classification rules that are tripping up people trying to defend Hillary Clinton using the “it wasn’t classified at the time” defense. If the months and days on the dates don’t match exactly, and there aren’t precise 10 or 15 or whatever year differences you’re only seeing the date when the material was stamped classified, not when it actually became classified. Since classification is determined by the nature of the material, not when someone stamped it as such.