From what I've heard, the stereo mixes were made rather quickly, so they would have a stereo mix and could fold it down to mono if necessary. If they'd made the mono mix first and run out of time, they wouldn't have had a stereo mix at all (which, IIRC, was advertised quite clearly on the posters). So they got the stereo mix done quickly so it would be done, then spent more time on the mono mix to perfect it since that was what most people would see. I'm not too sure where the six-track fit into all this, though.
And I've heard the mono mix (I have it if you'd like me to send you the links, Gaff) - it's fantastic, and (in my opinion) superior to the stereo mixes I've heard. I have never heard the 1977 theatrical stereo, though, just the 1985 and 1993 mixes (though I have all three).