It is because China hate Mongolia and consider nomadic people barbaric, and plus they claim Mongolia is part of China. But the reason why I said it is strange question is that Mongolian people don’t hate Russian people even though they annexed Mongolia during Soviet time and killed a lot of Mongolian people and still claim that Mongolia is part of Russia. read more
So I go on youtube, different forums online, and like, every, and I mean EVERY Mongolian person I encountered hates Chinese people. Just google the two words "Mongolian", and "Chinese" together and you will have a good idea of what I mean. read more