They took this same 4 channel mix for the 70mm 6 channel mix, but this is not the equivilent of today's 5.1 split-surround mixes. In 1976, Dolby Discreet 6 channel sound was introduced which consisted of left, mid left, center, mid right, right, mono surround. In 1977, they introduced Dolby "Baby Boom" 6 channel sountrack, which consisted of left, center, right, mono surround, 2 LFE (Low Frequency) channels. This is what Star Wars was released in.
Remember, theatrical 70mm with magnetic 4.1 channel/6 track presentations was preferred over theatrical 35mm 4 channel/2 track optical presentations at the time was because optical soundtracks did not have the fidelity of magnetic soundtracks, which were only available on 70mm. Since then, 35mm has caught up with 70mm, so it is used less. Home presentations are not going to present a substantial difference in using the 70mm 4.1 channel/6 track over the 35mm 4 channel/2 track mix. Most home systems will decode the original 4 channels from the 2.0 tracks.
In short, 2.0 Dolby Digital is perfectly adequate. We are talking about our desire and demand to have the film as it was in 1977 without subsequent tinkering, it seems a bit much to demand a modern tinkered 5.1 soundtrack.