First of all, the main reason is that China's economic center of gravity has shifted to the south. Before the Sui and Tang Dynasties, China's economy was still in the north, which was the most active area in China. Chang 'an, the capital, was the pillar of the northern economy and was extremely prosperous. However, after the Sui and Tang Dynasties, the population in the north gradually migrated to the "barbarian land" in the south, and the economic development in the south gradually surpassed that in the north. China's economic center gradually moved south, and the northern economy began to decline.
Second, Chang 'an is located in the northwest of China. Compared with the southern region, the land is relatively barren and the rainfall is less. The growth of local crops is limited and cannot support the development of megacities. Before winter, all places can transport food through rivers like Chang 'an, but in winter, when the river freezes and the river freezes, Chang 'an will fall into food shortage. Emperor Wendi of Sui once took Chang 'an officials to Luoyang to find food because of the lack of food in Chang 'an.
Therefore, after the Tang Dynasty, Chang 'an could no longer support the development of big cities. In fact, during the period of Wu Zetian, I knew that Chang 'an was the short board of Beijing, and Wu Zetian and others often lived in Luoyang. Chang 'an is just a spiritual symbol.