I think it's because Terry Pratchett offered something rare. Neil Gaiman gets at it in his wonderful revelation "Terry Pratchett Isn't Jolly. He's Angry." Reading his novels, particularly once Discworld was firmly established, that is easy to see. Carpe Jugulum, Monstrous Regiment, Thud, and Snuff are filled with the same sort of anger-inspired satire as Jonathan Swift's "Modest Proposal."
In an age when anger overtly fuels politics, religion, and in far too many cases, social media, destroying trust and severing ties that should connect, Terry Pratchett offered the sort of laughter that comes from recognition, laughter that makes sometimes bitter truth much easier to swallow. He reminded us that human beings are strange and complex creatures, simultaneously marvels and dupes, fearful and brave, petty and honorable. What distinguished his heroes from his villains was that his heroes were capable of rising above their less admirable impulses and doing what needed to be done, even as they recognized the cost.
His Death, who always spoke in small caps, claimed him. He might have been ready; we were not. We needed that voice. We still do.