From Sam in the comments:
In addition to what you’ve mentioned I think the reason for a lot of the downplay is that Communism is on the “other” side, and that the Left reflexively defends and gravitates towards anything Anti-Western/American/Conservative.
It’s the mark of a simple mind (and something I see from the Right as well) to feel like you MUST defend anything that the other guys hate (or the converse). Hence the hilariously dissonant defense of Islam, or a radical open borders position, or demanding that abortion be celebrated not merely tolerated, etc.
The Right’s sudden love affair with Putin is another example. In fact, Russia is basically a ball that is batted between the 2 major US ideological factions as needed.