1. The rise of mass media, and a mass media culture that promotes scare news.
2. The rise of social media which allows people to tailor their content to what they want to hear, including conspiracy theories
3. A general trend over the last 20 years of Americans becoming more isolated in their own communities from one another, leading to distrust of others
4. Various individuals and groups have an interest in keeping the US government divided and gridlocked to avoid legislation damaging their interests.
5. Trump. Trump has promoted this, directly and indirectly, throughout 2020.