There's a selection effect at work. It seems as if everything leaks because you don't hear about the stuff that doesn't leak. Plenty of things have been kept secret for decades: differential cryptanalysis, Ultra, FOGBANK. Those are just some of the ones we know about. It's quite likely there are more we don't.
Yes, they were kept secret for decades and yet you know about them now. It's not always a question of how long can information that is considered potentially dangerous can be kept secret. Sometimes it's a question of can it be kept secret indefinitely if the information is deemed dangerous enough. Many would say no.
