An alternative question is: Why not more? There are approximately rational scales with more than 12 notes. Something I wonder is how the complexity of music rel

An alternative question is: Why not more? There are approximately rational scales with more than 12 notes. Something I wonder is how the complexity of music relates to its use. For instance, instruments for music that's primarily ceremonial, or used in centralized locations by trained experts, could adopt scales with more notes, or more difficult tunings. This includes 12TET, which was difficult for an untrained musician to replicate, and unlikely to stay in tune for an entire performance on some instruments. Simpler tunings might lend themselves to instruments that were homemade, used for folk music, carried by travelers, played at home, etc. In fact those two things could coexist within a single culture. There were pipe organs and folk fiddles in Europe during the same time period, after all. Once the 4 strings are tuned by means of an easily discerned interval, you can fill in with a tolerable scale by ear. "Carried by travelers" suggests an advantage for a tuning system that can be restored by a non-expert and used for music that spreads from town to town.