There are a very few theories that have massively changed man's understanding of himself and the world around him. I suggest that Marx's works and Darwin's works have respectively played a major role in social theory and science. So then Peterson asks why the postmoderns choose a Marxian approach over some other basis (and he does not specify what basis), I think it's because these thinkers realize that they cannot avoid confronting Marx despite the horrific history of applied Marxism. The French Revolution's principles suffered a similar fate in the Reign of Terror.