Why did the Tang Dynasty govern the country with Taoism?

At the end of Sui Dynasty, Taoism said that "Yang must die, and Li Bixing". At the end of the Sui Dynasty, Shi Mi and Li Gui, the leaders of the rebel army, all claimed to be emperors because of their surname Li, and many ordinary people followed suit. After the establishment of the Tang Dynasty, Li Yuan regarded Li Er as his ancestor, called himself a descendant of Laozi, worshipped Laozi as a "holy ancestor" and took Taoism as the state religion to raise his family status. This is very common in history, such as Gao Qi Emperor Xiao Daocheng and Liang Wudi Xiao Yan. After their ancestors were unknown, they all pretended to be Xiao He of the Han Dynasty. For example, Du regarded Du Yan, a famous official in the Northern Song Dynasty, as his "direct ancestor".

Monarchical power is given by God, which creates the legitimacy of its own political power in the name of Taoism.