Despite HTTPS adoption rising, there are still more - major - sites without it. If you're the maintainer of such site, what's holding you back?
Google's report on top websites using https <a href="https://transparencyreport.google.com/https/top-sites" rel="nofollow">https://transparencyreport.google.com/https/top-sites</a><p>An argument I hear is that ad networks don't support https well enough. (I don't have websites with ads so I can't tell).