When people talk about implementing HTTPS they are primarily talking about implementing it on a server. But HTTPS makes demands on the content of pages too. You can't use a <script> or <link> element to point to an HTTP page from an HTTPS-served page. If you try, the browser will refuse, with a message like this:
Mixed Content: The page at <URL> was loaded over HTTPS, but requested an insecure script <URL>. This request has been blocked; the content must be served over HTTPS.
Nothing wrong with this design, but if you have to go back in time and retrofit HTTPS on a site that was initially created to be served with HTTP, you very likely will either have to convert a bunch of sites, or move things around. In my own setup this alone would result in a lot of breakage.