For example, one of the biggest CDNs in the world (I'm looking at you, Akamai), charge dramatically more for delivering content over HTTPS. Let's say you're delivering video content at scale, the difference between HTTP and HTTPS delivery can be many millions of dollars a year.
But why not use a different CDN, say Cloudfront which prices the same for HTTP and HTTPS?, well, simple, the same problem. Cloudfront is many times more expensive than other CDNs at scale.
Really, we need to apply pressure to all CDNs to equal out their HTTPS pricing (Its not just Akamai...)