Quick outline of how it all went to shit:
1) Cable penetration grows exponentially during the 1970s
2) CNN starts, gets huge ratings, spins up CNN2/Headline News
3) Other companies want a piece of the pie, including Rapert Murdoch
5) Turner, and other owners realize it's pretty fucking expensive to spin up fully-staffed, fully-equipped bureaus around the world, and fly crews to big news stories on little/no notice
6) Turner, and other owners realize it's far cheaper to do less news, and more long-form interviews with "newsmakers"
7) Turner, and other owners see ratings hold steady or even increase
8 ) Turner, and other owners realize keeping eyes glued on the channel makes as much money as growing ratings
9) Turner, and other owners realize the viewers most likely to stay glued to the channel are the more extreme, rabid, fucking idiot ends of the political spectrum, and start booking interviews accordingly
10) Fucking idiots stay glued to the channel which resonates loudest in their echo chamber.