Postmodernism rejects any theories of the world that have any kind of absolute structure, while marxism is explicitly about defining such absolutes. In marxism, all people are either workers or capitalists (or outside the system); any good has an intrinsic value, and any work has an intrinsic plus-value; and on and on.

You can also see this in the art favored by countries which liked to claim to be marxist-leninist (though they weren't). Far from being post-modern, it was in fact traditionalist and realist.