Why was the northern economy strong before the Three Kingdoms?

The Yellow River Basin was originally the birthplace of China culture. At that time, most of them were farming economies, and people at that time were also very simple-minded, and they immigrated when they had food. The fertile land and rich products in North China Plain naturally become the first choice for immigrants. So Xia, Shang and Zhou all built their capitals in that area, and the natural population was prosperous. By the Warring States period, the Guanzhong area where Qin was located at that time, that is, today's Xianyang area in Shaanxi Province, was no longer the barren Loess Plateau today. At that time, there were grass and trees, and the grain was abundant. What's more, Xiaoshan has a natural barrier of Hangu Pass, but the six eastern countries can't beat it, so people can live and work in peace and contentment. In order to escape the annual wars in the six eastern countries, people immigrated to Qin in large numbers. Driven by Shang Yang's political reform, Qin's economic strength was greatly improved, which laid a solid foundation for unifying the world. Later, Han and Chu fought for hegemony in the world, but Liu Bang still killed Xiang Yu's strength by virtue of Guanzhong's advantages of being easy to defend but difficult to attack, rich in resources and large in population. After the founding of the People's Republic of China, the capitals are still in Chang 'an and Luoyang. At that time, the Yangtze River basin in the south was still a barbarian land, with complex terrain, inconvenient transportation, poor information exchange and many miasma epidemics. The Chu state here should maintain good relations with local ethnic minorities all day, which is not suitable for a large population to live in. Therefore, although Chu has a large territory, its economic strength is always weak.