The program detailed in the bombshell Washington Post report wasn’t meant to be publicly disclosed, despite the fact that it had been established in 2007.
A government whistleblower provided the information to the Post because of the chilling capabilities of U.S. government surveillance.
From the Post report:
Firsthand experience with these systems, and horror at their capabilities, is what drove a career intelligence officer to provide PowerPoint slides about PRISM and supporting materials to The Washington Post in order to expose what he believes to be a gross intrusion on privacy. “They quite literally can watch your ideas form as you type,” the officer said.