I think people often overlook the “RP” appeal of things. I put it in quotes, because I don’t mean actual RP, but rather the aesthetic and feel of things. Even if you aren’t into the setting on a high level, people still choose their race and class based on flavor to a large degree, and the truth is the Horde just has more flavor and variety than the Alliance does.
If you go all the way back to the start, there is far more variety in the Horde than there is in the Alliance. The core Horde races are vastly different from one another in how they look and how their cultures are presented (the Native American themed Tauren, the Caribbean themed Trolls, the noble savage barbarian Orcs). Even the “human” race of the undead are unique, both in their moral dilemma of re-obtaining free will and because they have that outward “evil” vibe superficially.
The Alliance has no flavor. The humans are generic medieval, neutral/lawful good heroes. Dwarves, to a degree, are just short humans with a few exaggerated features who have indistinguishable motivations from the humans. Gnomes are even shorter humans with a few exaggerated features who have indistinguishable motivations from the humans. Night Elves were the sole outlier, but their presentation in WoW has lead to them being more homogenized as well. I suppose the burning of Teldrassil has given them new motivations, but before I couldn’t really distinguish their goals from their allies.
It got slightly better through Cataclysm. Draenei are visually distinct and very unique, but at their core their motivations and approach aren’t terribly different from their allies. Worgen are distinct with a different background, but at their core they are still human. On the flip side, Blood Elves and Goblins offered a very different dynamic to the core races of the Horde, both visually and culturally.
Allied Races are a mixed bag. Both sides have their “knock offs” that feels more like subraces. Heck, every single one except Kul’tirans (who are still just humans) have a core race in their name, relying upon an adjective to differentiate them (Highmountain Tauren, Mag’har Orc, Dark Iron Dwarf, etc.). Still, the same trend continues, where the Horde simply has more flavor to their additions more often than the Alliance. I think it boiled over with the Vulpera/Mechagnome addition for this very reason.
Tl;DR: The Horde is a confederacy of very distinct, unique races in appearance and motivation. The Alliance is, well, an alliance of very similar races with virtually no differentiation in motivations. In classic RPG terms, there is so much more story potential and feel when painting on a neutral canvas that a stereotypical good one.