Another part of the problem is that Obama promised fundamental changes, made only some minor improvements, then the party took a significant rightward turn (especially in foreign policy). His signature domestic policy achievement was so minor that healthcare was the biggest issue in the next two Democratic primaries. His two biggest foreign policy achievements (the nuclear deal with Iran, beginning to normalize relations with Cuba) were immediately undone by Trump, and Biden took no steps to revive them.

He overpromised and way, way underdelivered, often with a notable lack of trying. You can't sell people on incremental change if you build only small things the other guys can immediately break, and if you don't even try that hard to do better.