The Mongol Empire rapidly expanded to become the largest land empire in history, but then it disintegrated just as rapidly. What caused this so rapid demise of the world's largest pre-British Empire?
The reason is that it was too large to be governed effectively. Already Genghis Khan understood this when he split it to uluses and assigned them to his sons. It was supposed that the Great Khan in Karakorum will perform the general governance. The system was not effective. When Mongol armies broke to Poland and Hungary, they had to interrupt their conquest because the great khan died, and the leaders had to attend the kurultai. (Which normally took years).
It was Khubilai Khan who in a similar situation decided to continue his conquest of China rather than go to the kurultai. So he created a dynasty in China which ruled almost a century. Another big piece of the empire, the Golden Horde, existed till 16th century.
I'm not sure the "disintegrated so rapidly" assertion is correct. As Alex answered, they dispersed intentionally (into Siberia, China, India, Persia).
Second, "so rapid demise of ..." is also relative. Kazakh Khanate lasted until 19th century. Both co-founders of this Khanate (khans kerei and zhanibek) were direct descendants of Genghis Khan. In short, Mongolian empire (incl Kazakh Khanate) lasted till 1847. That's over 600 years.
What I am really getting to is that these statements about the Mongols has been around for a long time. These statements don't help to understand Mongol history because they are simple throw-away lines which feeds into the popular myth. They are usually to the effect: "that they disappeared quickly, or the Mongol empire disintegrated, or they came and went like a flash ..."
Let me finish by saying, actually, I recognised this question immediately because it's from a famous article "The Mongols: Ecological and Social Perspectives", Harvard Journal of Asiatic Studies, Vol. 46, No. 1 (Jun., 1986), pp. 11-50. It isn't really a question, but more an assertion.
From the very first paragraph, in full (emphases mine):
"FOR all that has been written about the Mongols of the twelfth to fourteenth centuries, some fundamental questions continue to intrigue us. What set them in motion? Why did they wreak so much more destruction than the other nomad conquerors who had preceded them? Why did they become Muslims in Muslim lands but not-apart from a few individuals-Confucianists or Taoists or Chinese Buddhists in China, or Christians in Rus'? Why did they stop when and where they did? Why did their empire disintegrate so quickly?"
Much research on Mongol history has been done since then, over 30 years of additional research. I do not understand why readers here are still adopting a Western-perspective from 1980s and assuming it is still correct.
The "core" Mongol group was too small, relative to its empire. Even today, "Mongolia" has just over 2 million people. In 1200, it was more like 1 million, with an army of 100,000.
Compare this with the fact that China had about 100 million people, Persia, perhaps 20 million, and Russia, perhaps 10 million (population sizes vary depending on how these "countries" are defined). That's too many people for 1 million Mongols to control.
In theory, the Mongols could have used "proxies," but they were culturally backward, and could not control more advanced and populous nations, except by force. Only Genghis Khan and his immediate successors could do this, and the longer the length of time after his death, the more Mongolian control was lessened.