When Bood Elves entered the scene, the Horde was comprised of lumpy malformed hunchbacks. Horde but not dysfunctionally hideous? Of course people signed up in droves. That was the entire point, really - the Vanilla Horde didn’t have a sustainable population.
Yeah, you’ll have your loud, scream-y fans and nostalgia drunk hipsters who to this day refuse to accept the Blood Elves as Horde, but the numbers don’t lie. No matter how many spikes you put on your mudhut and how guttural your screams of “lok’tar ogar,” the vanilla Horde was always going to have niche, limited appeal. For most, “lumpy, savage hunchback” is a Halloween costume, not a fulltime identity. Those people are just loud - not important.
Also, be honest - the Blood Elves aren’t just Humans with pointy ears - they’re better proportioned, idealized humans with pointy ears. The humans of WoW, at least the men, are all lumbering shaved gorillas. Even if you wanted the “bland, human experience,” the ultra-stiff, roided out humans aren’t really all that appealing of an option. Not in the face of the Blood Elves.
As for the Night Elves, even Blizzard sees them as the red-headed stepchild of the Alliance. Their design was always slapdash - seriously, you can find articles and interviews from the designers of WC3 who said they made them purple for, “some reason.”
Why are Night Elves the colors they are? For the lols, apparently. It’s another non-standard, weird form that’s always going to have that caveat. Compared to a race that doesn’t, that has broader appeal, the numbers will skew accordingly.
As for the Void Elves? … They were a mistake, plain and simple.
Blood Elves simply hit that perfect sweet spot of design, not too weird, classically beautiful, a hint of edginess for to pander to that crowd, deep roots to WoW Lore, and just a whole lot of appeal all around. Why the numbers are what they are isn’t hard to see at all.