Because of the music notation survives even without orally passing down. Church music or sacred music plays a big part in this. Because Churches took music a lot serious than folk music, and monks and musicians took the time to transcribe music into written notation. This notation passed down and people can study them systematically, and that is how music theory developed. Today, Blues, Jazz, Rock or even Metal, all adopt the music theory, and that is how things get perfected. China on the other hand, has no systematically preserve its tradition music, and hence, lack of sophisticated development.