They Knew More, Yet Keep It Simple

The ancients strongly disliked this tuning impurity of the fifths because it went against their mystic ideas.  If you believed that your God was up there on the nearest hill listening to the music you were playing, you were going to be jolly careful that you didn’t offend Him. The last thing you would want to do would be to deliberately choose some hideous irrational number to represent the frequency ratio of a flattened fifth, rather than the primitive integral beauty of 3 to 2 for a perfect one (the fundamental frequencies of the notes forming a purely tuned fifth – no beats - are in the exact ratio of 3 to 2).  So they decided to make music using fewer than twelve notes to the octave, even though they knew all about the twelve note scale.  Hence those boring and funereal old modes which characterised music until about a millennium ago, at least in Europe.  The restriction to 6 or 8 notes to the octave made modal music-making pretty simple during all that time.

Quoted from Temperament: A Study of Anachronism by Colin Pykett