1. Harmonic intervals in 12TET are good enough to sound okay with no obvious sour notes.
2. Hand and finger size.
Factors and primes aren't the issue - it's how closely the octave divisions approximate perfect whole-number interval ratios.
12TET hits the sweet spot. The tuning is close enough for practical keyboard and fretted instruments to cover a wide pitch range but still fit human hand and finger sizes. So they're not impossibly difficult to make, tune, and learn, but you can still make complicated music on them.
Finer subdivisions like 19TET - or more - are closer still to the ratios, but you need computers and electronics to make practical instruments. Or highly skilled string players:
1. Harmonic intervals in 12TET are good enough to sound okay with no obvious sour notes.
2. Hand and finger size.
Factors and primes aren't the issue - it's how closely the octave divisions approximate perfect whole-number interval ratios.
12TET hits the sweet spot. The tuning is close enough for practical keyboard and fretted instruments to cover a wide pitch range but still fit human hand and finger sizes. So they're not impossibly difficult to make, tune, and learn, but you can still make complicated music on them.
Finer subdivisions like 19TET - or more - are closer still to the ratios, but you need computers and electronics to make practical instruments. Or highly skilled string players:
https://www.youtube.com/watch?v=MPHLS5mJrJk