The limitation was the hardware. Computers were not able to handle full raw film scans until recently. In 1993 I bought my first new computer with an impressive 420 MB hard drive and 16 MB of ram. I did an impressive upgrade to two 800 MB hard drives and 32 MB of ram in 1997. Using PNG compression just the titles and opening crawl of A New Hope would fill those two 800 MB hard drives at 1080p resolution. So in 1997 it took massive computing power to just scan the segments they did scan for each film in the Special Edition. No home machine could handle it. In 2004, things were better, but still, storage did not really allow for filming or scanning movies at such high resolutions. Today it really isn’t a problem and they do it on a regular basis. 2k captures everything the average viewer is likely to ever notice. 4k captures everything a discerning viewer is likely to notice. Double that for anything shot on 70 mm (the original negative was actually 65 mm - the extra width is for the magnetic soundtrack) and double it again for anything on IMAX. The original negatives and even a first generation print (usually studio presentation prints and the distribution interpositives) maintains much of that quality (Citizen Kane’s negative was lost long ago, but they did a 4k scan of the print they have for restoration).
With hardware no longer the issue, what becomes important is the original intended resolution. I think Lucas did a great job on that because there is hardly anything that shows up on the blu-rays. The worst I noticed was some traces of blu-screen (I noticed this in 1997 in the theater while watching Empire and I think it is on the blu-ray). I’m not sure if much more would show up at 4k. With such effects heavy films I worry that increasing the resolution will just make the composting artifacts more visible. But Star Wars is already ahead of most films. Many have noticed things in some movies at 1080p that the photoghaphic print process rendered invisible. From the Team Negative One Silver Screen Editon, I did some comparisons between it and the blu-ray to try to figure out the resolution of the details and I found no difference between the SSE and the blu-ray reduced to 50% (960x540). Now, the primary source is a copy of distribution print, so there is bound to be some quality lost in the copying process. But Harmy is working on enhancing the scenes for a full 1080p Despecialized edition, so they are considerably better (in most respects) from the GOUT, so it can be upscaled a bit. His works gives hope that we could see 4k versions of Eps 2 and 3 that wouldn’t look worse than the originals. When you combine the quality of that scan (along with the Empire and Jedi grindhouse versions) and the variables in projection, you get a distribution print quality that isn’t much better than DVD’s. Especially not crip ones played on an upscaling blu-ray player on a 1080p screen. That level of quality hides a multitude of sins that 4k scans reveal in stunning clarity. Star Wars doesn’t seem to have many such sins and I wish Disney would do a quality 4k restoration of the 4 35mm films.