It is because early postmodernists found a "Truth". And by that i mean a postmodern version: a reality from their point of view. Their critics of power, art and knowledge were right... Half a century before they made them for physics, and decades ago for biology, but for other fields (except maybe philosophy that turned around a bit before).

This did not take in scientific fields because they did their revolution early XXth century. And while postmodern critics were right, their proposed solution (relativism and/or nihilism) weren't. Everybody now knows that when you have a valid critic, your solution will likely be viewed as "more valid", even though this is absolutely untrue. That's probably what happened in social studies.

Also i follow subects in humanities (history and linguistic, and some consciousness philosophy) and those follow the same epistemological methods. Sadly you can only find the papers in academia and at least in France, know "historians" (TV historian) are not following those methods at all.

ps: The only postmodern i read was foucault, so everything i've said on postmordernist only apply to my understanding of him.