There were well-defined legal channels available to Snowden to communicate any concerns he had about the existing programs. By all accounts, he did not take advantage of any of them.
As dissenters within the agency, they have a more accurate idea about how all of this works in practice than nearly anyone else.
Also this has been going on for so long, why didn't anyone else from NSA use said well-defined legal channel to expose NSA activity or at-least put them up for scrutiny.
I can think of 3 reasons:
1) There is no well-defined legal channels to expose NSA illegal activity, to the extend Snowden did.
2) NSA only hires people who are morally corrupt.
3) NSA employees are genuinely concerned about their safety and well-being if they want to expose NSA.