It's definitely related to the revocation of the fairness doctrine. Watch some newscasts from the 70s on YouTube [0] and compare to now - it's like night and day. Sure, the news was a lot more boring back then, but that's probably how it should be. Certainly sometimes there would be opinion, but it was clearly demarked as being the opinion part of the broadcast and it was towards the end of the broadcast. You could watch the 6PM news on ABC, NBC or CBS back in the 70s and they were each going to be covering pretty much the same news items in pretty much the same way.

[0] Here's one from '79: https://www.youtube.com/watch?v=uvX25IG4tvY