Well, sound mixing is usually so subtle that 19 out of 20 people don't ever notice, so theres less pressure or incentive to get it that exact. As long as its faithful to the effect and intent of the original and sounds the same people not only don't mind, they don't notice. I think a lot of the time since mixers have to go back to the stems or original elements anyway they think why not sweeten it up, since theres usually room to improve in the original mixes with the new technology available, and the changes are so minor that the original four or six-track mixes become superfluous. At the end of the day it usually becomes unnecessary, except in the cases of original mono or stereo tracks that sometimes are offered (more for novelty than anything else). If you included a new mix plus the original multi-channel then you'd have to sacrifice picture quality--which people do notice.

Star Wars is a bit of special case though, not in the least because the six-track mix is a bit of a legend itself, but also because its a distintcly different version of the soundtrack, along with the stereo and mono versions. It would be awesome to have all three on a disk of the original 1977 version of the film, and the six-track, though perhaps not as nice as a new 5.1 mix of the original would be for home theater, would probably suffice as a high-quality multi-channel version of the film. Would be awesome to have all three original mixes plus a new 5.1 but I'm not sure if they could fit in on a disk without taking away something from the image. I would say though that because of the demand and notoriety of the 1977 mixes that theres a very good chance we'll see these with a restored 1977 version of the film one day.