Because moderation here works quite well and they usually aren't the ones being moderated. Like someone wrote earlier in this thread, why aren't the free speech advocates using less HN to debate hacker/tech stuff and using more 8chan or something?
I think that probably stems mostly from Hacker News avoiding controversial subjects in general. When they do slip through, the moderation can be pretty bad, particularly when it comes to new accounts. I've seem innocuous comments shadowbanned for voicing fairly milquetoast heterodox opinions (with shadowbanning in general being a pretty unpleasant action). Usually that isn't an issue with accounts that have been here for a long time, but since Hacker News doesn't allow people to delete comments, you have to be comfortable with having that comment tied to you for decades to come (not always the safest thing in this environment).
The whole thing ends up exerting a chilling effect on alternative opinions.
Assuming the content is the same: why is censoring a prominent voice worse than censoring someone less well-known? Prominent voices inherently have greater platform access, professional clout, etc. Whereas lesser-known figures do not have such privileges and are therefor more greatly impacted by censorship.
Prominence just makes it tougher for the platform to avoid ambiguity in their justifications. Some moderation is valid but other times it can be too much. None of us have the time to spend personally reviewing the claims of every contributor who feels like they were treated unfairly.
We see this with Twitter now. People complain about various tweets being blocked, and there's always a "back-and-forth" about how Tweet A is against their guidelines but somehow Tweet B isn't. But when they outright ban (e.g.) Donald Trump, there's no ambiguity any more. The discussion moves beyond the minutia of spam / bot handling and into something more concrete.
Though you're right: censorship of those with smaller voices is at least as problematic. We just all ultimately have limited resources available and focusing on the more clear-cut examples is more likely to be successful.
This site self-selects. People who think the moderation is good stay, people who think it is bad leave. There are plenty of alternatives to this site, you can easily find one that suits your needs. It's not an effective monopoly like the big social media giants.
