Answer:
The answer is The lack of a proper and organised system of administration and the lack of a clear succession model.
Step-by-step explanation:
By the time of Genghis khan's death, the Mongolian empire was the largest empire in the east, spanning from Mongolia to the eurasia. However, after the Mongolians completeded the invasion of China and persia, the empire became the largest land empire in history and it was evident that a traditional emperor based system cannot control or sustain it.
Further more, many battles over the succession ensued in the later years of the empire, dividing the Mongolians and decreasing their military and economic strength. As the Mongols were primarily many clans unified under a one banner, this struggles of succession was commonplace and eventually it affected the empires downfall as well.