Postmodernism gained traction as a critique of high-modernism, viz. the broad idea that the natural and social world can be reliably understood through large, complex theoretical frameworks and that this extends to remolding them more-or-less arbitrarily in a way that will create a kind of technological, economic etc. 'progress', 'evolution; or 'growth'. If you think that 'large, complex theoretical frameworks' are mostly BS and inherently subject to all sorts of observer biases, etc. that's at some level a postmodern idea, even though other broad viewpoints (such as pragmatism) would also agree with this.

The "postmodernism" that's criticized today has drifted quite a bit towards high-modernism itself. There was a shared sense when this first came up that you can only critique "large, complex theoretical frameworks" by building a new large, complex theoretical framework of your own. But ultimately it turned into a key weakness.