Dolby 70mm incorporated the old Todd AO system for 70mm playback of Left, screen left, center, screen right, right, mono surround.
Instead of using the screen left and right channels as mixes were done from a LCRS four track master-Dolby took the low end and specifically mixed it to come out of the unused two screen channels and eventually some theaters moved placements to become in effect the first subwoofer in a modern theater context. It’s sort of confusing as to how these tracks should be presented in modern codec to be accurate: 4.0, 4.1 or 4.2.
This was done partially to compete against Sensurround which was attempting a comeback and other systems as mentioned above like Fox’s own Sound360 and Warner’s Megasound which was supposed to have some pretty impressive low end.

Apocalypse Now in 1979 was the first release to have split stereo surrounds but only in its 70mm version. This is why it is said to be the first 5.1 track in theory. Superman tested this a year prior but that mix was not released.

Mono was still dominant and it was not thought that Dolby would be widely accepted especially since there had been many format to come and go quickly. Stereo was still not very well utilized and most theaters would do it badly. That’s why Kubrick stuck with mono. On ESB Ben Burtt and others specifically mixed the Dolby stereo to have all the relevant information in the main two channels so that if anything went out you could still enjoy the film and not lose anything. Dolby Stereo was an unknown factor and not expected to take off like it did. Thus the mono mix was done last for the general main release and intended as the final definitive mix.