What is the reason for the formation of the tide of respecting meaning in calligraphy in Song Dynasty?

Respecting the law in the Tang Dynasty made calligraphy study have rules to follow, but it also exposed its weakness, that is, it was too rigid in the law, which was not conducive to the display of personal artistic interest. Especially in the late Tang Dynasty, its weaknesses were more fully exposed. So people did the opposite, and the trend of "respecting meaning" developed. The pioneer of Shang Yang's political reform was Yang Ningshi in the Five Dynasties. Yang Ningshi is a transitional figure, and his works contain the strict spirit of laws and regulations cultivated by the Tang people. At the same time, with the help of his pretentious behavior, he added lyrical factors to the book. However, all these inspired Su Shi, Huang Tingjian and Mi Fei, and made calligraphy in the Song Dynasty become a trend of respecting ideas.