The 6-track is obviously the superior format, and it wouldn't take much to transfer the original elements to a 5.1 mix while remaining true to the opening day
I thought they were all released in mono. And also since most theaters (at least at first) likely projected the film with a mono mix, that was Burtt and Lucas' preferred soundtrack since that was the one they invested the most time in.
Learn something new every day, I guess. I'll revise my sig accordingly.