Reverse psychology against hackers/spies. Let them think that it was a super obscure, difficult code to crack. Nope, according to the article it was listed in

Reverse psychology against hackers/spies. Let them think that it was a super obscure, difficult code to crack. Nope, according to the article it was listed in the instructions handed out to the soldiers who manned the silos. If there had been post-it notes back then, they probably would have stuck one right on the console with the code.