One could just as easily say that it's almost as if the Democratic Party is designed to absorb and diffuse leftist impulses, leaving room for a party to carve off leftist voters but never enough to seriously destabilize two-party rule.
Blaming a small handful of goofballs, true believers, and opportunists for their effect on an entrenched network of powerful elites with the nominal backing of millions of people requires ignoring the power dynamics.