Modernism itself rejected historical progress, undermining postmodernism’s distinctiveness.

Evidence

‘Incredulity towards metanarratives’ seems therefore to be at least as old as the Enlightenment which was so productive of grand narratives in the first place. The fin-de-siècle recognition of what Sorel called the illusions of progress seems especially embarrassing for those who want to associate distinctively Postmodern art with this incredulity. For the leading figures of the heroic era of Modernism at the beginning of the century generally rejected the notion of historical progress. — Alex Callinicos, Against Postmodernism, 61