1, Mongolia's only valuable thing is minerals, and Mongolia's ore can only be sold to China. Mongolia cannot air transport ore, and Russia does not buy ore. Let Mongols to mine minerals can avoid many troubles, such as environmental protection, minimum wage and so on.
2, The income of Mongols is less than 10% of that of Chinese people. If Mongolia becomes a Chinese province, the Chinese government needs to spend a lot of money to help the poor.
3, After the disintegration of the Soviet Union, the relationship between China and Russia has been very good. Mongolia's strategic position is only useful when China wants to threaten Russia's Siberia, and we don't want to make the Russians feel threatened.
4, The most important point is that the Mongols are unwilling to accept PRC rule, and China is unwilling to repeat the mistakes made by USA in Afghanistan.