I think people seriously underestimate how reactionary the American voter is. As bad as the Democrats are, more of our problems are demand-side than we want to admit.

To expand on this, we've had centuries of white supremacy that prevented a labor movement on par with those in European social democracies from emerging, the US never existed in the shadow of communism, and the reactionary/neoliberal capture of major media has created an extremely reactionary proletariat in the US. Add in electoral systems such as the Senate and Electoral College that prioritize the votes of low-education white people and you get an electoral politics dominated by reactionary voters.

While the Democrats are still an awful, incompetent party, the reason no left wing party (either a third party or a more left Democratic party) emerged in the US is because there is very little demand for one.