Answer:
There are two major reasons
The first factor was Jin Dynasty interference in steppe battles and politics.
The second factor in Genghis Khan's prowess and that of his descendants was the need for the spoils of war. Due to the fact that the Mongols were nomads, they wanted to enjoy some of the features of a settled society, which included silk clothing, fine jewellery, and more. To maintain the humility of his ever-growing army, as the Mongols conquered and absorbed neighbouring nomadic armies, Genghis Khan and his sons had to continue to empty these cities. His followers were rewarded for their valour with luxurious goods, horses, and slaves they forcefully took from the cities they had conquered.