A lot of what he does involves mixing in lower quality sources, but making it look seamless. And I’m not talking about the GOUT, I’m talking about stuff like cloud background plates on Bespin, explosion elements in the Battle of Hoth, new CGI that takes time and resources to render at higher resolution, etc., not to mention newly filmed live action elements. These elements blend well enough at 720p, but at 1080p the seams start to show (I believe he did tests for ESB:R to determine this), and it would just look bad at 4K.

For what he’s trying to accomplish, 720p is what he chose to commit to as the most ideal balance. 1080p will not happen, let alone 4K.