system
Given the following post, respond as if you're commenting on Reddit:
human
[Warcraft] Why was there a 10,000 year gap between the first and second Legion invasions, but only a few years between the second and third?
gpt
The night Elves were the dominant, and only noteable race on Azeroth at the time. While the Naga had been corrupted (somewhat) and heroes like Illidan had fallen, the demons had been vanquished with the well of eternity sealed, which was their gateway into Azeroth.

It took 10,000 years for Sargeras to find an individual powerful enough on Azeroth that he could corrupt to open a new portal. With Mannoroth having corrupted the Orcs and making a portal from Draenor to Azeroth, he was given a first chance to conquer the planet, as the Orcs seemed to be on the path to integration into the Legion proper, though they were repulsed twice before the Legion itself was brought in.

The latest invasion seems to mostly be triggered by the time traveling/universe hopping Gul'dan from WoD, the legion has a number of factions in Azeroth now they can call upon for support to lead another invasion, something they didn't have after their initial invasion, and the 10 or so years since the 3rd war the Undead Scourge has fallen (Ner'zhul/Lich King definitely did what he could to undermine the initial invasion even sending Arthas to lead Illidan to the Skull of Gul'dan and kill Tichondrius. ) and the various other tragedies and infighting amongst the powers of Azeroth has effectively weakened them or at the very least prevented them from building their forces like the legion has been doing so.