You may have fallen prey to the 'HN commenters are in SV' fallacy. HN long ago became an international community, and even in the US its readers are mostly outside SV. The majority of HN commenters aren't that well-disposed toward SV.

Users here have a broad range of politics, just like any sufficiently random large population sample. The ones who feel strongly about X are more likely motivated to post to X threads. So what you're seeing is not a high communism rate in general, just in (lord help us) communism threads. In other words the sample-bias fairy is afoot.

To round out the bias list: a good test when you think you see a pattern on HN is to ask if the same is true of humans in general. We're seeing a return of classic ideological battles (e.g. communism vs. capitalism), but that's also true in society at large. It's the macro trend; HN just gets wet in the same rainstorm. I guess history didn't end after all!