Because it is a live tape of the events during which the first nuclear submarine attacked, fired on and sunk an enemy warship, then successfully evaded the escorts. As well as the tape there is avery laconic written commentary, explaining some of the noises heard on the tape. It would give any author an insight into the cool and calm professionalism of an attack. You could probably get this from some of the YouTube videos available. I also had the chance to listen to a US one, which I refused, as there was a running commentary by the very brave XO until crush depth.
So far as I know, they are not in the public domain - but who knows these days!