What are the reasons why Mongolia has a low population?

The birthrate of mongolian people is not that great. it might be related (in parts) to the fact that it is well seen in Mongolia to be drinking a lot / getting drunk, which can then lead to less effective reproductive rate.

also, being nomadic people (for most of the citizens, at least), there is need for larges spaces to move cattle around.

Another hint could be the way the country is not so hospitable. there are not much natural ressources, life is harsh, and most people are below poverty line.