But the point is that the end result of the project of modernity had resulted in the development of the means of atrocity, of automatic mass death and holocaust, and the only sane move in that instance was to put it aside and move past it. Hence: "post".
Coupled with the burgeoning theories from physics, Heisenberg's uncertainty, and the rest, and the idea that perhaps we not on as solid a foundation as we thought, and the time was ripe for moving on to new ways of thinking about the world.