1. The reason is very simple. There were many calligraphers and works produced at that time. For example: Cao Cao, Zhong Yao, Wang Xizhi, Wang Xianzhi, Wang Huizhi, Suo Jing, Wei Heng, Mrs. Wei, Xie An, Wang Xun, Bo Shaozhi, Xiao Sihua, Xiao Yan, Wang Hui, Wang Sengqian, Wang Zhi, Wang Ci, Yang Xin.
The formation of regular script, Wei stele and stone carvings. There are a lot of them at this time.
2. Social background: the development of productive forces, especially the economic development in the south; the stable social environment in the south; the climax of national integration; the blending of multiple cultures.
The history is very unfamiliar, but I hope it helps.