Consider for a moment why he was kept in prison in virtual isolation until the end of his life, whist Albert Speer, Hitler's architect was rehabilitated after the war and became rich on re-telling his stories of life inside the Nazi cabal. Hess was effectively a traitor to Germany and you would have thought, a friend to the Allies, whist Speer continued almost to the end in his support of he ever increasingly mad Hitler, only parting during the last hours of the Berlin Bunker.
No, there was something nasty, closer to home, that Hess represented, and he could not be allowed to be free to tell. My guess is that the ever crazy House of Windsor is involved somewhere.