Short story version is, very few theaters had Dolby back then, and there were technincal problems with mono playback of a Dolby Stereo track, (now no longer the case) hence the need for a mono mix.

The 70mm mix was probably rushed a bit to meet the release date as well.

Lucas and company had no way to know if Dolby would be a short lived technological gimmick or not. Stereophonic sound in movies had never really caught on beyond the few huge venues equipped for it. Therefore, the mono mix was considered "definitive", and more time was taken with tweaking it.