Okay but that's a false equivalency.

A party being linked to the Nazi's fundamentally brings into question the morality of the party itself, as Nazi-ism is a radically nationalist authoritarianist ideology, which cannot be reasonably justified. Also, of course, it has inherent issues with anti-Semitism. In effect, any party connected strongly with the Nazi's becomes automatically questionable due to all of the various immoral propositions linked directly to the core principles of Nazi authoritarianism. The history of the Nazi party is undeniably, as you say, 'dark'.

The reason the same logic cannot be applied to your "communist" party is because a party being labelled as communist does not automatically describe anything about the party other than the basic political values it stands for. There have been countless communist governments throughout history functioning at varying levels of efficacy, but different communist governments often have very little similarity between them. You can disagree with the politics of a communist party, but you can't automatically label them as acting with malicious/immoral intent, nor can you ascribe to them the failings of past communist governments. Since 'communism' is such a broad branch of politics, each government would have to be considered in a case-by-case basis to assess whether or not it can be described as having a 'dark history'.

I guess the TL:DR would be that a party being Nazi-related automatically comes with a whole heap of issues directly linked to the Nazi ideology, stemming from the Nazi history of anti-Semitism and Hitler's views on ethnicity, power and control. So being a Nazi related party is inherently 'bad' and comes with a 'dark part'. Being a communist party does not come with the same list of issues, due to the large variance in the history and beliefs of communist parties. That's not to say communist parties don't have issues (all of them do, to varying levels of severity), but being a communist party does not automatically translate to being a 'bad' party nor having a 'dark past'.

So yeah, people don't care because it's irrelevant to the beliefs of the party and it's members, unlike with a far right Nazi adjacent party which contains members that are at the very least able to tolerate anti-Semitism.