IMHO, the Vietnam war radicalized media ... thing is, I wasn't politicized myself until adulthood, so I may have been missing the biases.
That and Watergate, where the media got their first real taste of the power they had by bringing down a President they thoroughly hated (I'm not making any excuses for Nixon or what he did, but can anyone doubt that if the press had rallied around him like they rallied around Clinton, he would have easily survived Watergate?).
I wasn't really all that political myself until the Clinton years, but even as apathetic as I was in the '80s, I couldn't miss the hatred and contempt for Ronald Reagan that dripped from every word the media wrote about him. But that was a different era, post Vietnam and Watergate, so the media spent those eight years actively looking to bring him down like they took down Nixon, which necessitated letting their bias show.