Why are the capitals of China in Xi during the prosperous period?

It should be said that when the People's Republic of China was founded, the national strength was very strong, and then it gradually weakened. The so-called world is not the name of one family. Geographically speaking, Chang 'an and Xi 'an. There is a natural barrier between Hangu and Wuguan in the southeast. There are scattered customs in the west. As long as we control Hanzhong, we can stop the invasion of Sichuan. Sichuan uprising and rebellion can't be achieved, and Hanzhong has no control antenna. In many dynasties, feudal rulers transferred Hanzhong from Sichuan to Shaanxi. Prevent Sichuan insurgents from establishing a separatist regime.