You know your history well, but the last several decades of media manipulation was done under the guise of the myth of the “honest reporter”, set forth by Hollywood in the 1930s and 40s and people like Edward R Murrow, Walter Cronkite, and David Brinkley.
Most Americans in the last half of the 20th Century fell for it, and we're still suffering the ramifications today. It would be far, far better for us if we did go back to the openly partisan coverage of yesteryear. The modern myth is a great falsehood!
The problem you describe came about as broadcast media came of age. The timeline is identical. Along about the same time small newspapers were being bought up by large conglomerates. These days small independent newspapers are scarce as hen's teeth. It started first with radio networks. Guys like David Sarnoff at RCA controlled the airwaves. Then, as television came along media was further consolidated into essentially 3 major outlets. People gradually stopped reading newspapers and began getting their news from the "big three". Uncle Walter was born. Eventually, we saw competing newspapers in larger cities, the one semblance of balance in media, fall one by one. The result was as you describe.
The Genie (the Internet) is out of the bottle, though, and the Ivy Leaguer's suddenly don't like it at all. There are now hundreds of sources of information beyond the big networks. The "big three" are losing viewers and listeners at staggering pace and they know it. Listen to all the cat calls to shut down the "new media" and the blogosphere coming out of mainstream media. If they could compete with ideas they would, but they can't. History and facts aren't on their side, hence the charade they've been putting up for decades. If you listen closely you'll hear the big media tell you exactly who threatens them the most......