HTTPS requires an SSL cert that requires to be setup on the website. Usually these are quite expensive. But with the recent (2016 I think) release of mozilla's letsEncrypt certbot, one can install FREE SSL certs on a website.
Additionally, there can be issues when loading a webpage over http and resources over https and vice versa.
I can't remember the exact figure but I know a huge number (the majority) of websites don't actually have SSL on their websites.