Why did the Mongol empire split?

when Genghis Khan died...

The Mongol Empire was divided into four sections by Genghis Khan.

The Empire was to large to be maintained otherwise.

To much conflict would have been caused by a single heir being named.

In truth Genghis Khan's sons were not worth much and his daughters and wives did the real running of the Empire.

The sons and daughters in law did not like this and after many years of fighting pretty much ruined the Empire Genghis Khan died ( Old, in his bed surrounded by family, how many rulers had that claim to fame in ancient history).