By this thought process, could it maybe be that the Soviets were waiting for the Americans to reveal they knew something first? I imagine the secrecy would make the threat of their use seem more likely, and thus a stronger deterrent once eventually uncovered by their enemies. It added to the "crazy enough to do it" factor even into today. On top of that, a huge smoking gun like that could even serve as bait for enemy spies to potentially reveal themselves. I really do find it hard to believe that no other country caught a whiff of any of this, save for the 1979 anthrax outbreak.if the Soviets could hide it so easily surely the Americans must be, too.
The Cold War is one long, epic tale of second-guessing. The fundamental basis of US foreign policy for 40 years was a 5500 word telegram saying, in effect, "the Soviets will never see reason, box 'em in so the infection doesn't spread."