They didn't. The truth has been hiding in plain sight. But it's been masked in conspiracy theories, kooks, swamp gas, ridiculed and spun to the point where; if a UFO did land on the WH lawn, most people would think it a Spielberg creation.
WELL PUT.
Stanton Friedman deals with the secrecy stuff very handily on his website.
http://www.stantonfriedman.com