The SV atitude on communism threads is always interesting. Communism is great, quit whining you greedy doctors. Helping people is its own reward! But for me? No no, I'm a developer! I deserve my salary!
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, and HN can't be immune from that. We get wet in the same rainstorm as everyone else. I guess history didn't end after all!