You might be right about the digital mastering--now that I think about it, my 1993 VHS says "digitally mastered" on the side, I am pretty sure. I guess it's really just the THX process that makes it unique. Personally, though, the films looked sharper and cleaner, especially where dirt and grain were concerned. This could just be tricks in the telecine, I suppose.
As for why they didn't use a restored OOT built for the SE, the restored version of Star Wars wasn't quite so step-by-step. It wasn't like they restored the whole film from start to finish and then did the enhancements. The enhancements were being done at the same time the restoration was, as the pieces were ready, so there was never any moment where there was a full film of restored footage, it was always a piece here, a piece there. ESB and ROTJ were not even part of the plan in 1995, those SE's were only decided only around 1996, so the restoration of those was done after the THX videos were already on shelves.