If you want people to act, you need them to care, and if you want them to care, you need to get them riled up. If you want to start a social movement, or bring about social reforms, you need to make people feel a sense of urgency. Moral outrage, not bare facts, stirs people to action. I cannot think of a single significant social reform that was not born of a deep sense of dissatisfaction with the status quo.
Thus, progressives reject the story of progress not because they reject the facts it entails, but because they see it as a threat to future progress—because they think that the only reason we have come this far is because we have resisted concentrating on our achievements. They don’t hate progress: they just hate talking about it.
If this is correct, we are left with a paradox: in order to achieve progress, you have to downplay (or even deny) its existence. This would explain why so many progressives have trouble listening to claims about how good things are, or how much better they are than they used to be. While they might be empirically accurate, such claims sap the energy we need to keep moving forward.