It's more philosophical than political. Since the dawn of Modernism at least, there has been a very strong misanthropic streak in philosophy. After Postmodernism took off, it's just been at the forefront, something which even the meta-modernists and their ilk aren't quite ready to abandon.
I don't know, I see "postmodernism" in popular literature to be a more descriptive term than a prescriptive one; "postmodernism" might be able to tell us that literature has tended to be more cynical in this selective way but won't really tell us why. Very few authors are going to set out to write the Great Postmodern Novel simply for the sake of doing so, and most of what we expect from postmodern literature has its roots decades before anyone thought up the term (like you mentioned, Kafka's probably the most influential writer in the field, and IIRC works as far back as
Don Quixote have been slapped with the postmodern label). It's also not like works that aren't in the framework we've been describing can't or won't sell well and be highly regarded by their contemporaries (Arthur C. Clarke hardly went hungry for not writing like Phillip K. Dick, for example). In all respects, it seems like authors can break from postmodernism very easily if they decide to, so it's a question of supply and not demand here.
I think it makes more sense to call the trend a political one. Most of the groundbreaking seminal work in creating misanthropy in fiction where there wasn't any before had a fairly explicit political agenda (
Dark Knight Returns, I'm looking at you), and most of the political utopias from the period tended to decide whether or not humans were bastards based on the politics driving them (for example,
Ecotopia is firmly on the Humans are Bastards side; anything by the Strugatsky brothers, not so much).
And to a large degree, I think OP is over-observing things, or thinking that this is recent when it honestly isn't. Most fiction that has survived to the modern period is fairly misanthropic, even if it does have an optimistic ending. This is because either, without people doing horrible things to other people there is no plot (and thus it's all fucking pointless,) or frankly, the work grasps some deeper universal truth, generally relating to how people are pretty horrible to each other and life isn't all that great.
The difference, I think, is that past work identified "people doing horrible things to other people" as an aberration rather than as the natural order. Sure, the factory bosses might be horrible now, but that's only a temporary state of affairs until we unite the workers in the inevitable revolution! The fuzzy-wuzzies in Darkest Africa might be cannibals now, but just wait until the Emparh brings them the light of civilization! The Galactic Emperor might be a total bastard, but once we throw him down a reactor shaft everyone will get along fabulously!