in short because it was so systematic and widespread that it is unparalleled i history. Hitler also made it clear he only started it because he wanted to control people. It runs on that spectrum: first they came for the Jews, then they came for the gays, then they came for the disabled, then they came for me. He was running through the motions of systematically running through the entire population of everything on earth one at a time to re-engineer the wold according to his own ideal. We already know the Jews would have only been the tip of the iceberg and he had the same lined up for every race on the planet: every minority, every dark skinned person, every non-Germanic, every sick person, every disorderly person, the elderly and the unborn. He was systematically using totallised state control: data control, Identity control, birth control to systemically ID people into specific categories and push the world into an endless world of legitimized genocide. I don't think Americans realize this is why the EU is so gung ho about data protection. Hitler had this exact identification system lines up on the cards with the aim of wiping out enormous swathes of the Earth. He would never have stopped f he had won, and he was always completely unrelenting and unapologetic until the very end. He pretty much invaded Russia because e wanted to kill as many people as possible while he could. He got impatient and wanted more death sooner.
Added to that Hitler transitioned from being a global icon, to a bother, to a threat, to an enemy, to a tyrant, and all the while whle he did all that the world had no idea what he was actually doing. By the time Hitler had reached Tyrant status he was already he entire world's enemy. When the holocaust was recovered it revealed how naive they had been to believe his lies, and that anything that had transpired, and everyone who had died fighting him, had no idea what they were actually dying for. They knew it was a form of evil but no one suspected it was on that scale. They suspected it was happening but thought his desire for total annihilation as some crazy pipe dream he'd never try. Even as he was being defeated he diverted resources into exterminating more people. That tends to be why the world said never again, when it comes to believing funny haired racists making claims about restoring national pride.