If the later Gregorian chant developed out of Old Roman and Gallician chanting, what happened to the more sophisticated musical features of those earlier traditions? Old Roman chant features things like background harmonization as well as more sophisticated melodic structures which seem to have disappeared in Gregorian chant. Does anyone know of any historical reason for the simplification of liturgical music and the eventual victory of the Gregorian tradition?
P.S. Not sure if this would better in a subreddit devoted to music, but I'm more interested in historical, social or political aspects of the question than the music theory.
By "background harmonization," do you mean drones and parallel movement? A lot of chant "harmony" is not actually notated in the music and is created by modern interpreters. I am not sure the Gregorian tradition is less sophisticated than the Old Roman and Gallican traditions, and it did absorb some of those traditions. The Gregorian tradition became dominant largely because Charlemagne required it, in an attempt to consolidate and unify church traditions. Gregorian chant also coincided with the notation of music so that it could be consistently disseminated instead of communicated solely through oral tradition.