https://www.troyhunt.com/https-adoption-has-reached-the-tipping-point/
I think Let's Encrypt is a big factor.
I've moved over one major site and two small sites in the last nine months. The major site was a ground-up rebuild, though there were a few hiccups. The others are just Wordpress sites and it's just a matter of hunting down a few mixed mode issues. At this point, for a Wordpress site, start to finish should be under a couple of hours on a server with Webmin installed. You just click a button to create and install the Lets Encypt cert and it's done.
The biggest factor, though, in terms of number of sites (rather than amt of traffic) is CPanel integration. Like Webmin, you can change with a couple of clicks. Unlike Webmin it utterly changes the server, so shared hosting providers can be slow to do major version upgrades. So even if the newest version of CPanel has added Lets Encrypt, that might leave a lot of people out. Not sure, but assuming most people have Lets Encrypt with free certs and pushbutton install on CPanel, I'd things to really grow