It isn't the base itself that's so secret...
it's what goes on there. Area 51 (Groom Lake) has become so widely known that some people speculate it really isn't being used much anymore except as bait for the rubes. They claim the real black-world work is taking place elsewhere. As for Groom Lake being so super secret, I've been reading about it in open aviation histories for many years, especially in regards to the U-2, Blackbird, and F-117A programs. The things that are tested there most often enter the white world within a few years. The longest held secret that I can recall from Groom Lake was about 10 years, and that was pretty much an exception.