We only have relatively quick access to artwork if the user is using SD. With DVB/Freesat/XMLTV epg it's more complicated and slower to find the artwork. It's a lot more straight forward in the code to have the application behave the same way, regardless of where it's coming from. There is also a lot of people with rubbish internet, and a lot of these image files are pretty large, so regardless of whether there is a good CDN it can be slow to download these images.

That said, I'll put it lower on the list of things for possible inclusion in v5.