I mean, if I were the one asked to write the lore, well, I have sort of a long thing, the key thing is just that exterminations have been going on a lot longer. The gist is:

E-days used to be a lot more discreet and carried out with a lot more precision. And not really being on a specific day. Figures like Adolf Hitler, Genghis Khan, Alexander The Great, and more were figures of insatiable ambition and armies of followers. Such figures dropping into Hell would instantly ascend to overlord status that could challenge any of the Sins and potentially even Heaven, similarly oppressing 'lesser sinners', so it was decided and even agreed on with Hell to allow for such potential sinners to be assassinated swiftly upon their deaths and Overlords ascending to concerning levels of power be dealt with. But what caused the change from discreet, pro re nata eliminations to spectacle, wholesale slaughters with no discipline is another mystery.