Why did calligraphy in the Tang Dynasty have its heyday characterized by "attaching importance to law"?

The Tang Dynasty was the heyday of feudal society in China, and calligraphy also appeared the heyday characterized by "attaching importance to law". Four great calligraphers appeared in the early Tang Dynasty, such as Yu, Chu and Xue, and Li Yong, Yan Zhenqing and Liu Gongquan.