Well this is more of an opinion question, than a factual one. But truthfully it has to do with the fact that Mongolia after Genghis Khan (and his successors), did not affect the "West". And hence Western historians don't bother with it. If you were reading Chinese history, Mongolia would pop up more often, even after the collapse of the Yuan Dynasty.

So it depends on your point of academic interest. But we have to remember that "World History", as a topic of study, was only "created" in the later 20th century, and was not so much about the "World", as much as it was about the "World's" interaction with the "West". This has been changing though, but in many cases it is still true.

Because Mongolia wasn't a "thing" outside of Genghis Khan's empire (and the modern state and its antecedents, of course). Genghis's empire was only one, albeit the most succesful one, of many different "Steppe empires" that have existed for as long as recorded history, from the Iranian Scythians to the Seljuk Turks. Central Eurasia has a way of cultivating these "empires that periodically burst onto and overwhelm the agricultural, sedentary states along their fringes. If you study Chinese history, dealing with these empires have been more or less the central political question of every dynasty.