Two things: specificity and deliberateness. When the Final Solution was put into action at the height of WWII, it was with a view to wiping out entire classes of people – Jews, gypsies, homosexuals, and the disabled, to name a few.
The Nazis undertook this work with a level of planning that was unprecedented in human history and has never been matched since. They made an earnest attempt to kill every single Jew in occupied Europe using all the techniques and technologies of the industrial age.