Marc Ambinder explains why:
Though the less-painful penalties for being caught – shame, loss of a security clearance, suspension – might deter some would-be leakers, whistleblowers tend to get away with it. The law is not straightforward, and when you combine it with perverse incentives inside the intelligence community and with a cannon of decisions (formal and informal) that defer to the prerogatives of major news entities, it tends to provide a measure of protection. That’s not to say that the government isn’t trying. They just aren’t very good at it.
The Dish 