That is because Mongols are basically steppe warriors who refused to give up their way of life to efficiently govern their empire. However, it helped them to preserve their identity and survive as an ethnic group until today. They did not blend into Chinese society and did oppress them ruthlessly, for example. They maintained that distance between them and their conquered subjects, which foster increased resentments. So eventually they were thrown out by locals as more or less intact ethnic group.
All steppe nomads, not just Mongols, had real contempts for civilized way of life as corrupt, cowardly, and lazy. In their views, they were sheeps to be slaughtered for nomads' benefit when they need extra food or any material. They were not about to adopt the life-style of hapless sheep. They genuinely believed that their way is superior because they conquered them handily. Nothing else mattered. Turks and Manchus also had to grapple with this issue of how much assimilation they will accept. I think that Turks were more sucessful than Manchus. Manchus were melted into the ocean of Chinese.
It never does. Those recessive genes will do you in every time.