Why did the ancient Han people "stop making progress" after occupying the agricultural land in East Asia? Instead of developing in Central Asia and South Asia?

The Han nationality is a farming nation, and the areas where agriculture cannot be carried out will not stay long even if they are occupied by the army. Even if it is not a large number of immigrants in the late Qing dynasty and the early Republic of China, it is hard to say whether it can be guaranteed.

Huang Renyu has the concept of 38 mm isohyet. In the southeast of the isorainfall line, the average annual rainfall is at least 38 mm, which is suitable for developing agriculture and has a prosperous population. The northwest of isohyet has a dry climate, which is suitable for nomadic people to live on aquatic plants. In bad weather years, nomads like to invade the south.

Although Southeast Asia is close, sea routes still have great risks in ancient times. Moreover, the climate of Han army is not very suitable for tropical climate. The Central Plains Dynasty fought many wars against Vietnam and Myanmar.

As for South Asia, the ancient road is difficult and the army can't go.