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.
