Most HN users didn't sign up looking for "Respectful discussions on controversial topics" (and aren't seeking that out), and of course the site wasn't designed for that. It's like me asking why HN isn't a good place to debate atonal music theory: no one else wants that and the site wasn't designed for it.

If "Respectful discussions on controversial topics" is sincerely what you want, check out https://old.reddit.com/r/changemyview/ . There, folks did sign up for that and the culture and moderation was built around it. They also set and enforce rules designed for that use case, like "Explain the reasoning behind your view, not just what that view is (500+ characters required)" and "Only post if you are willing to have a conversation with those who reply to you, and are available to start doing so within 3 hours of posting."

It takes quite a bit of work to do well: https://www.reddit.com/r/changemyview/wiki/rules

You can read or join every phrasing of every conceivable vaccine topic: https://old.reddit.com/r/changemyview/search?q=vaccine&restr... (or search for related keywords).