The U.S. mass media, dependent upon the Executive for their information and careers, and run by corporate interests benefiting from Executive largesse, predominately convey Executive Branch perspectives on an hourly basis to the American people. Even on the relatively few occasions when they publish information the Executive wishes to keep secret, it has little impact on Executive policies while maintaining the illusion that the U.S. has a "free press". The U.S. Executive is essentially free to conduct its activities as it wishes.
~ from America's Most Anti-Democratic Institution: How the Imperial Presidency Threatens U.S. National Security, by Fred Branfman ~
Again, as in the previous post of this miniseries, it basically boils down to economics. The mainstream media can't survive without advertisers or mainstream backing. If you challenge the powers that be too consistently and/or too forcefully, the money dries up and you're out of business. So, you play along by the "rules" and, if you are fortunate, the money keeps flowing.
Besides, the corporations who now own the vast majority of the mainstream media are members of the elite anyway. They certainly don't want to support anything that will hinder their ability to ride the gravy train. This is why you rarely see their reporters ask tough questions of the government or other elites. They only want us to know what THEY want us to know.
Every now and then, a major story or scandal blows past their pre-designed boundaries (like the current NSA brouhaha). In these situations, circumstances force them to do something they really don't want to do: report on the underbelly of the beast. They tend to do so in a manner that seeks to minimize the damage as much as possible in the hope that it will blow over quickly which will allow them to move it off the front page or to train their cameras on some meaningless spectacle.