The simplest version of the postmodernist argument is that grand, overarching narratives are just pareidolia for academics - people search for shapes where there are none; and that perception shapes reality to such a degree that one cannot examine simply the ‘conditions’ of a society, even non-material ones, and expect to understand its contours.
It’s more nuanced than that I think, but honestly, it’s been almost a decade since I was last in college and read up on it, so I don’t know how much I trust myself to give a more in-depth explanation, lol.