This is much easier than fighting the Burning Legion back in the day. The Legion had a ton of players, so trying sneaky tactics rarely worked.
Sometimes, you'd even get counter-sneaked.
The reason this happens isn't complicated—Void faction monster templates are just way too damn ugly.
The Burning Legion's demons are also pretty ferocious, but at least they're dripping with badassery. The sheer variety of fierce and imposing demon designs has undeniable appeal for male players.
And then there are high-value species like the Aireda, Shivarra, and Succubi.
Especially Succubi—hugely popular. Their high facial customization makes even the plainest girls pick Succubi to create stunning looks. Add those explosively sexy figures, and Succubi are crazy attractive to both male and female players.
This leads to the Burning Legion faction being just as populated as the official faction, despite standing on its own.