It seems clear they don't want us to know something.
Now, it could be more or less political, or personal, or something ultimately mundane, but the conspiracy theorists can make it anything they want of it.
I suggest there might have been something really complicated wherein nobody could point the finger fully at any group, but that the CIA was doing something sketchy, funding the wrong people, Johnson new about it but only in superficial terms, someone somewhere took it too far, it got out of hand and that the truth is just too damaging to the state (and various agencies) itself so they want to keep it hidden.