system
Given the following post, respond as if you're commenting on Reddit:
human
[World of Warcraft] Why are the people of Azeroth successful in repelling and ultimately defeating the Legion when countless other worlds have failed?

Like...what's special about us? Did the other worlds just not try or what?
gpt
One answer is practice.  Prior to fighting the Legion directly, the residents of Azeroth fought among themselves *a lot*.  They also got plenty of practice fighting demons, fel magic users, undead swarms, dragons, shadow magic users, Old Gods, weird ancient bug peoples, Pandas' inner demons...boy it's a long list.

Another answer is allies and awareness.  The forces of Azeroth received help from people like the Draenei, who had encountered the Legion before, and the Army of the Light, who had been fighting the Legion for ages.  There's also Illidan and all his angsty warnings.

Yet another answer is that Azeroth has always been a distinguished place.  It drew attention early on for having a very powerful Titan soul growing within it.  Old Gods flocked to the world, and the Titans came to its defense. 
 Sargeras and the Old Gods/Void Lords have long been after Azeroth, seeking to corrupt and/or destroy the planet and its denizens.  The peoples alive on the planet today are survivors and heirs to a long lineage of fighting evil stuff.