If you're serious about your question, it's because the mass butchery associated with self-professed Communist regimes is a consequence of the state being hijacked by an authoritarian for their own ends - which, I'll be the first to admit, is a corruption that communism is particularly prone to - whereas the explicit goal of Nazism is to mass murder anyone not identified under the state's ethnic definition of nationality. Not as a consequence of anyone hijacking the movement, but as page one of their manifesto.
And to most people, that's worse. Much worse. Unintended consequences were at least unintended.
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