There are many reasons, such as: Tang and Song Ci works are mostly desk works, which do not need to sing or never sing, and do not need music scores; Songs without text symbols; Some lyrics may be sung "by the sound of words", without a fixed melody rhythm, and there is no need to write music; In a few cases, although there are written symbol music scores, they only circulate in teaching workshops and musicians, and do not or rarely flow into the market, so they cannot survive in large numbers and the probability of being handed down is extremely low; As a "simplified spectrum" with imperfect symbolic means, the music score of Tang and Song Ci can't replace the symbolic spectrum of oral speech, and it doesn't make much sense to mark the spectrum, so even the word set used for performance is not marked. Basically, ancient music was not composed, but was passed down from generation to generation and naturally disappeared. Moreover, people at that time did not realize that these concerts were in danger of disappearing, and over time, they were forgotten. My humble opinion. For reference only.