Also, nobody knew at the time if Dolby Stereo was going to be a flash in the pan or not. Other systems such as Sound 360, and Megasound both fell into obscurity.
Even something as reasonably successful as Sensurround was only used on four movies. Given the potential for structural damage in old theaters, we're probably safer now without it. ;)
Ben Burtt has been quoted as saying the mono mix was intended to be the "definitive" version.