It's because of the fragmentation of news sources that can now be customized to one's preference with any finely sliced political cant desired. Three television networks maintained a measure of neutrality in their reporting. Mainstream print media did too albeit to a lesser extent. Wackjobs had a hard time finding each other to form a tribe that would destabilize society.

Before the rise of social media, the mainstream news ignored both things going on in minority communities like police violence and ignored rural America. The MAGA community and Trump himself bypassed the mainstream media because of social media.

America has always been polarized - institutions just never gave them a voice.