I think the biggest reason why huge leaks like this don't often occur (despite the age/maturity level of the young enlisted personnel entrusted with the information) is that it's cool to have a Top Secret clearance. It sounds weird, but when you're having a couple beers with your buddies and one of them asks a question about your work, and you realize that information is classified above them, you feel pretty tingly once you reply, "Sorry, bro. That shit's classified." Not to mention the risks for divulging that information are steep enough to deter most would-be talkers: reduction in rank/pay, prosecution, dishonorable discharge, confinement, hard labor, etc.
(Active duty military here)