Why didn't the Tang Dynasty rule Yunnan?

In fact, the rule of the Han Dynasty in Yunnan was very strong, and a large number of Han people migrated. At the end of the Three Kingdoms, several Han people's Nanzhong (formerly Yunnan) surnames were formed in Yunnan, such as Meng Huo, who was portrayed as a barbarian in many film and television dramas, but was actually a barbarian Han nationality. For example, cuan clan, who ruled Yunnan for hundreds of years, and Bangu Ban Chao belong to the same family, which is also a typical example of the barbarization of the Han nationality.

Then why did the Tang Dynasty rule Yunnan so weakly? Because after the Yongjia Rebellion in the Western Jin Dynasty, the Southern Dynasties basically lost control of the South. During the Northern and Southern Dynasties, although the territory of China often included the south in the territory of South China, the south gradually became the nominal territory of the south. The real ruler of the South is cuan clan, whose 400-year rule by the Han Dynasty was completely barbarized in the Southern and Northern Dynasties.

Then came Shi Wansui's conquest of Cuan, and the imprisonment of Yunnan in the early Tang Dynasty. In fact, Yunnan, which was taken over by the Tang Dynasty, is no different from the newly conquered territory. The blood of Han people in the Han Dynasty is basically barbarians. With a population of only 10 million in the early Tang Dynasty, it would be great to control Yunnan. If it were not for the rise of Tubo, the population growth in the Tang Dynasty would inevitably lead to immigration to Yunnan and gradually become a county. However, the rise of Tubo made the Tang Dynasty bear great pressure from Yunnan, Jiannan, Hexi Corridor, Qinghai and Anxi to Wan Li, and finally Xuanzong had to support Nanzhao against Tubo. If it weren't for Yang's coquettish operation, this strategy wouldn't be too much of a problem. Before the Tianbao War broke out, Tang Jun had a large number of troops stationed in Yunnan, cuan clan contained Nanzhao, and Tang Ting directly controlled part of Yunnan's territory. However, Yang's coquettish operation led Nanzhao to annex cuan clan and expel it from Yunnan. The two natural forest protection wars consumed 200,000 troops in Tang Ting, which directly led to the outbreak of the Anshi Rebellion and completely interrupted the road of sinicization in Yunnan. The next time Yunnan returns to Chinese territory, it will be 500 years later.