A recent, small-scale analysis we ran showed that more than 80% of the HTTPS URLs that are eligible for indexing (i.e. no crawl issues, no noindex, all’s good), can’t become canonical because website owners don’t tell us about them. They use the HTTP variant in their sitemap files, in the rel-canonical and rel-alternate-hreflang elements, even though the HTTPS variant works mighty fine.
As per Gary:
If your site supports HTTPS, please do tell us: use HTTPS URLs everywhere so search engines can see them!