Edward Snowdon understood what would happen if he where to seriously try and push the issue internally.
The global surveillance network was a core NSA policy authorized at the highest levels. This was not simply some rouge agent or rouge department. Previous individuals have attempted to raise concerns internally and failed to achieve any change underlying policy. The NSA has even deliberately lied to congress on the matter.
As a contractor, he has no employment rights. Making noise would likely get his security clearance revoked and his employer finding someone else who doesn't have a moral problem with surveillance. It would also likely get himself added to the NSA watchlist.
As a pragmatist, his decision to publicly release records has successfully created enough political pressure for congress to at least review the NSA's policies. A cowardly little shit who was willing to risk everything on a high risk venture, with a very strong possibility of getting caught, that takes some major balls from someone who knows exactly what the NSA is capable of.