In Hamlet, why can the ghost not tell the secrets of his “prison house”?

If you compare what the Ghost says in Hamlet with what Jacob Marley's ghost says to Scrooge in A Christmas Carol ("Nor can I tell you what I would. A very little more, is all permitted to me"), you'll see that there is a continuing literary tradition of spirits being unable to reveal all the secrets of the realm beyond that of the living. It is prohibited and we are not told why, but the assumption is that an even more awful punishment might descend upon the specter who reveals more than is permitted.
However, in the case of the ghost of Hamlet's father, he does elaborate, telling Hamlet that the things he could tell, were he able, would be far too harrowing and hideous for Hamlet to want to know. And, since the ghost wants Hamlet to believe that that he is, in fact, his father's spirit, he spares him the gruesome details.