system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does Mongolia have such a low population?

I'm surprised that it has only 3 million people. Especially considering it is next to China who have well over a billion.
gpt
They were taken over by the Soviet Union who destroyed their culture, history, and natural resources leading to sharp economic decline.  Their birth rate declined after the 70s sharply but is now rising again (59% of their population is under 30, one of the highest in the world).  

They also at one point had more Buddhist monks per capita than anywhere else on Earth, 20%+ of the population.  Those guys don't have kids.  

The main reason is just that it's a small, landlocked country with economic troubles and not a whole lot of resources.  It's kind of in the middle of nowhere, not particularly close to urban Chinese cities, surrounded by Siberia on one side and the massive Gobi Desert on the other side.  All in all a recipe for basically zero immigration.  China is many times the size, and has lots of coastal areas that have spurred trade, immigration, and a powerful economy.