Because Nazism is attributed directly to, and only to, Hitler and his immediate cronies. The rationale is that Nazism must be a once-in-human-history occurance. It is not. It is no different than any other tyrant in history on any continent in any country. Nazism is looked upon as Hitler's personal brand of insanity that somehow allowed him to hoodwink the German people, and many rich and powerful American admirers. His ideology was socialist. His and his czar's vision of their place in the world evolved as their influence and power grew. It got easier and easier for them to take complete control of every aspect of German life. It didn't happen over night, or with one election, one set of laws. Frogs in the boiling pot.