First, on the critique postmodernists and post-structuralists make of left and radical politics:
One of the most moving narratives of modern history is the story of how men and women languishing under various forms of oppression came to acquire, often at great personal cost, the sort of technical knowledge necessary for them to understand their own condition more deeply, and so acquire some of the theoretical armoury essential to change it. It is an insult to inform these men and women that, in the economic metaphor for intellectual life in the USA, they are simply "buying into" the conceptual closures of their masters, or colluding with phallocentrism (p.5)On the blindness postmodernism has toward capital and capitalism:
It is as though almost every other form of oppressive system - state, media, patriarchy, racism, neo-colonialism - can be readily debated, but not the one which so often sets the long term agenda for all these matters, or is at the very least implicated with them to their roots (p.23)Of course, if you follow Jean-Francois Lyotard in ruling out systematic social theory one cannot even begin to get to grips with the eternally shifting and conflictual social system that is capitalism.
Lastly, on why (effective) radical politics necessarily resists postmodernism:
Radical politics is necessarily hierarchical in outlook, needing some way of calculating the most effective distribution of its limited energies over a range of issues. It assumes, as does any rational subject, that some issues are more important than others, that some places are preferable starting-points to other places, that some struggles are central to a particular form of life and some are not (p.95).It's no accident the most enthusiastic proponents of postmodernism I've encountered tend to confine their radical politics to the seminar room. It's not for nothing the German social theorist Jurgen Habermas (by no means a radical firebrand himself) has denounced postmodernism as a strand of conservatism.