The core mechanism of political polarization is that the internet inexorably pulled society out of its temporary state of mass media consolidation. For a couple decades, we had an unusual situation where a few companies ran mass media, and those companies all sort of agreed to toe the centrist consensus line politically. Now, we've reverted to something like what we had before radio, which was that socialists read the socialist newspaper, right-wing folks read the right-wing newspaper, etc., if not even worse -- some folks just got their news from the loudest partisans at the bar.
We've gone from an era where news distribution was fragmented because it was very difficult, to an era where it was consolidated because it was easy for large corporations only, to an era where it's so dead simple that anyone can do it, so it's fragmented again.
Legacy media folks hate this. They blame the biggest players helping people share fragmented media sources with one another, rather than recognizing the inevitability of this fragmentation, no matter what products people use to share news over the internet. They demand a return to elite consensus blocking extreme viewpoints. It is simply not gonna happen. That barn was always temporary, and it has collapsed around the horse.