It's all Trump's rhetoric, his attitude, his lying, peddling conspiracies, etc. Very few voted for Biden. They voted against Trump.
Trump would've won in a landslide if he just kept his mouth shut and stayed off Twitter. His personality turns so many people off and he may very well lose because of his divisiveness rhetoric.
Like has been said, the actual President doesn't matter/effect most people. State and local elections, and senate/house matter more to how it affects your life. A lot of people just want less of a Saturday Night Live skit for a President at this point.