Answer by Anonymous:
Because Hitler invoked national policy explicitly designed to kill people, i.e. the Holocaust. The 17 million figure is the broadest estimate of the number of people who were killed during the Holocaust (), including the historical estimates of six million Jews, two million ethnic Poles, about two million Romani, homosexuals, handicapped individuals, and religious and political dissenters, two million Soviet POWs, and five million Soviet civilians.
In contrast, the majority of deaths under Mao and Stalin's leadership were due to starvation from idiotic socialist policies. I'm not saying that they (Mao and Stalin) don't bear moral culpability for the people who died, but unlike Hitler, they didn't set out to kill tens of millions of people.
For example, the historical narrative of the Great Leap Forward in China (1958-1961) is that Mao ordered agricultural activities be abandoned in favor of more modern and profitable industrial activities such as iron and steel production. This policy led directly to starvation, which in the largest estimates, totaled approximately 38 million. Was Mao Zedong morally culpable for these 38 million deaths? I would argue yes. Were Mao Zedong's actions evil in the same way that Hitler's systematic extermination of millions of Jews, Poles, Romani, and Soviets was evil? I would argue no.