The UFO conspiracy propaganda was a win-win for the US military. It kept the Russians from finding out about the secret pre-satellite surveillance methods (high altitude balloons, U-2, SR-71, and eventually stealth tech) AND everyone thought they were badass enough to have some kind of alien technology.
As the Cold War was winding down, Groom Lake was used as an illegal hazardous dump (well, technically not illegal, as the military is exempt from environmental regulation). When the lawsuits of civilian employees dying from strange cancers looked like that would blow that cover, they hired a chump to talk about "aliens at Area 51" on Las Vegas TV, and the sheep ate it right up and continue to do so to this day.