I browse a popular gaming forum that has not implemented HTTPS. Chatting with the admin, the reason is simple: Ads.
We can definitely blame the ad networks. Some have switched, but many won't work on https, and websites relying on ad revenue must stay with HTTP or make less money with HTTPS-friendly ad networks.
This hasn’t been true for at least a year. There is zero difference in ad revenue for HTTP vs HTTPS now. Join any major online community for publishers and ask anyone if you don’t believe me.
We can definitely blame the ad networks. Some have switched, but many won't work on https, and websites relying on ad revenue must stay with HTTP or make less money with HTTPS-friendly ad networks.