Although many sites are implementing HTTPs URLs, over 80% of these are not showing up on the Google results page as a result of webmasters submitting the HTTP version of their site!

According to Gary Illyes announcement on Google+ the vast majority of HTTPs URLs are failing to be presented with the SERPs as HTTPs and being displayed as HTTP because of a simple webmaster configuration.
This was discovered after an internal analysis by Google of the URLs that are eligible to be presented as HTTPs, found that over 80% of them were neglecting to take advantage of this. URLs that are eligible for display as HTTPs, are those that do not possess crawl issues, and do not have noindex scripts or any other problems.

The Problem Lies in the Sitemap

Gary has suggested that the cause behind this widespread neglect is the webmasters’ use of the HTTP variant of the rel-canonical and the rel-alternte-hreflang elements in their sitemap.

A message from Gary: If your site supports HTTPS, please do tell us: use HTTPS URLs everywhere so search engines can see them!