I think the issue in Missouri and Columbia were taken as such, and those were free speech issues. I think the bigger reasoning is that the mainstream is only going to report what gets it clicks and views, and those are the issues that gets it clicks and views - creating a self-reinforcing cycle. Hell, even TNC admitted that the Bernie article was absolutely clickbait.
The calcifying of sides in the media has been happening since the early 2000s - but now we're just seeing it happen everywhere because most of those large media conglomerates have started picking up the sites that used to be independent. Theoretically independent and impartial news sites now openly lean one way or the other, and court consumers that are ideologically aligned to them, while publishing articles that those ideologically different from them will use to mock and get angry about, but still give them clicks and views.
If you haven't ever gotten a chance to read it; read "Trust Me, I'm Lying". Pretty interesting peek into how much of this works (on a more general level than just identity politics).