[Chock]

It was probably kept secret because of the D Notice system in the UK, and possibly similar security systems in other countries, which are often allied to the D Notice system.

The D Notice system - an abbreviation of Defence Notice - in case you don't know it, is (or rather was) an official request to news editors not to publish details of certain events which it was deemed would affect national security. That includes reporting stories that are likely to affect the morale of the public, which means they were very often a request to the press to 'hush things up' so people didn't get upset. D Notices were first issued just before the start of WW1 by the (then) War Office (now the Ministry of Defence), but they were only requests, and not legal constraints. However, it was usually the case that editors would abide by them, not least because many of them eventually hoped for various things such as knighthoods, peerages etc, from the British Government, and needless to say, if they didn't 'play the game' they'd damage their chances of that happening. Since by their nature, D Notices are not officially binding, it's rather vague as to how long they are in force, so they tended to prevent some stories coming out for a long time.

Although D Notices are not legally binding (unlike court reporting restrictions) breaking a D Notice can land a newspaper editor in hot water just as if it was a legally binding demand, and make things uncomfortable for an editor's career prospects. When I was a writer at a UK daily newspaper a few years back, the editor of that paper let such a story slip out, and he damn near ended up in prison because of it.

More recently, D Notices have been dropped and are now replaced by Standing Defence Advisory Notices to the Press, which are permanent requests not to do stories, or make mention of certain subjects. Typically this includes things such as, the UK's nuclear offensive missile capability, where any secret bases are, anything to do with codes, or the secret services such as MI5, MI6 etc, as well as where high profile people in the Government live. All that kind of stuff.

There were several major ship sinkings during WW2 which were the subject of D Notices, notably the sinking of the liner Lancastria whilst it was evacuating people from France to the UK in 1940. This was kept quiet because an estimated 4,000 people were killed when it went down, and right after the Dunkirk evacuation, it was deemed to be bad for morale in the UK to report such a story. Similarly, because the Rohna was one of quite a few ships sunk by advanced German guided missiles, it was often deemed to be bad for morale to concede that the Germans had the capability to knock out large ships with just one aircraft launching a stand-off attack.

Al