To my understanding, it comes down to resources and rendering times. He does alot of modification to those films, and they take forever to render even just at 720p. 2160p would be just insane.
He discusses a few things in this post from 2012. Now that 4K77/83 exist, the 2006 bonus discs are no longer an issue, and I imagine whatever PC Ady has nowadays is perfectly capable of 1080p compositing. I guess at this point it’s mostly to keep things consistent with ESB:R, especially since AFAIK Ady’s still using the 2011 Bluray sources rather than the recent 4K releases, which are lower detail and so don’t take as much of a hit by being downscaled. Anyway, none of this is relevant to the thread, apologies.