I don't really know much about the specifics of Dolby Stereo optical in particular, but two-channel audio generally works reasonably well downmixed to mono. It isn't optimal, of course, because the levels may not come out right, and most especially because the matrixed rear channel effects will phase cancel out and will not be heard at all. So things like the blockade runner coming from the back before appearing on screen in the first film would be missing on a stereo mix played in mono, as would various other ship flybys, the chasm voice echo, and numerous background sounds. There are quite a lot more rear effects than you might imagine without hearing them on a surround setup, though not as numerous or immersive as present day mixes tend to be.

When they say Dolby Stereo is designed to be mono-compatible, does that mean that it has a way of ensuring that the surround effects will still be heard? I'm not sure how such a thing would work. Based on that alone I can see why they wanted to make a separate mono mix, and also to take into account the Academy EQ curve for mono setups without noise reduction. I guess by the time they made Empire and Jedi they assumed most theatres had upgraded to stereo and didn't bother making another separate version?