Forums, like social networks, are networks effects businesses. HN grew from a seed set among YC people, applicants, pg's essay audience, likeminded individuals. After there is a community and conversations, they pull in other people. (I randomly got drawn in when someone referenced my blog some years ago, and I just happened to stay. etc, etc)
Assume you have the magic wand of product design. You wave it and create The Best Forum Ever. That's great. I don't use it because I use HN and you have no reason for me to switch. People who want to read what I have to say can't find what I have to say on your forum, because I'm not on it. Sum up that interaction over the entire space of possible users and it leads to a forum with 0 users and no content.
If you (hypothetically) want to create a competitor, your big problem is not creating a better designed website. It is, instead, convincing a seed set of users to use your new website even in the absence of much content on it, and having those users be such wonderful/connected/popular/thought-provoking/etc people such that people will join your site just to hang out with them.