The declines could be due to price changes in the materials they used to print books, or a change in policy regarding how long to make them. It would not surprise me if they just allowed certain writers (Rutherford and Franz come to mind) to write as much as they wanted and publish whatever they wrote. As more writers started to work on books and needed guidance, they would probably have come up with a set of rules, including how long to make any books (which would also make it easier to get the presses set up and any other machines configured).
I like the thought that Franz was so humiliated by the 1975 failure that he lost a bit of the will to keep writing. But a guy in his position would have had an ego the size of the Sun. I doubt it bothered him to be so spectacularly wrong- he probably blamed Jehovah for welshing on a promise.