Because HN actually has the best content, it doesn't have to do any of the other things you mention to get users. So it can focus on the thing that really matte

Because HN actually has the best content, it doesn't have to do any of the other things you mention to get users. So it can focus on the thing that really matters, which is the value that users get from the site, which in the case of HN has nothing to do with graphic design or trend-following social features. If something is low quality, but many people use it anyway, that means that there is a part of it that is truly world class.