Why do you think it's easy for most to have nuanced, dispassionate discussions about the ancient Assyrians but not the Nazis? Spacial/temporal/ideological proximity. We're not likely to become anything like the former, but we fear ourselves (or at least our leadership) becoming more like the latter.

Obviously it would be ideal to learn from the good aspects of even the most evil totalitarian regimes, but in practice it's easier to wrap the whole nationalist/collectivist/communist idea-space up in a bundle and throw it out as one, at least in conversations with strangers of unclear motives.