While looking at various maps of some Chinese dynasties and geographic distribution of Chinese languages, I noticed that there's this narrow stretch of territory in the north-central-ish part of China that keeps appearing on these maps, as in the following images (all taken from Wikipedia):
What's up with this? Why did these dynasties expand their territory in this narrow stretch of land, while seemingly ignoring the land to the north and south of it? And how come Mandarin seems to be distributed along this stretch of land too (but not to the north/south of it)?
Geography, geography, geography.
The Tibetan Plateau sits to the southwest of that area, the Great Wall, Gobi Desert, Mongolians, and Muslim Uighurs are to the northeast, and the Silk Road runs right up that corridor. Without that area, the eastern dynasties would not have had control of the main trade route to Central Asia, India, and on to Europe. How much control they actually had is a matter of debate - the region is far from the eastern capitals, it's surrounded by hostile folks and, even to this day, the Chinese government has yet to truly pacify and incorporate the Uighur and Tibetan people into broader China.
This have been answered a few times before. Check this one by /u/AshkenazeeYankee