30 years ago in the US, there were a handful of major TV news outlets, and most people got their news from one of those and/or a newspaper, of which there were also a limited number.
The thing about those sources is that for the most part, it wasn't really economically viable to alienate half the population by leaning hard right or left. Any reduction in audience would likely translate to a commensurate reduction in advertising revenue.
Today, there are many, many sources of 'news' available in various forms around the internet, and of course people are free to choose what to pay attention to. This means it's entirely feasible for each source to cater to a particular viewpoint, even at the expense of definitely alienating half or more of the theoretical potential audience.
I theorize that the reason for this is that people have voted with their feet, balanced sources aren't as profitable and that's why there are fewer of them. It makes sense, a more balanced take on events is by definition not nearly as sensational, and almost always requires more mental effort on the part of the listener.
That by itself would probably be enough to explain the current situation, but on top of that, we also have the fact that many people receive the above mentioned information via algorithms designed to feed them more of what they already like (i.e. agree with) and nothing else, which of course only amplifies the effect further.
I have no idea how we get out of this situation (or if in fact we will), but in my mind it's not surprising at all.