Because the conquering Horde story is the most interesting Horde story.
- Players who want to play as evil beastmen have their fantasy fulfilled as they get questlines where they get to be the aggressors, invade neutral kingdoms, execute prisoners, commit horrific human experimentation, wield biological weapons, machinegun panda villages, and burn down elf cities.
- Alliance players get to play as heroes fighting against unambiguous evil.
The problem with the Horde is that it was sold as being Thrall's Horde from WC3, ie a reformed group of humane beastmen with morals. So you have a significant portion of the Horde playerbase who are hoping to get stories about good guy beastmen, which is irreconcilable with the above depiction of the Horde.
I've heard that while WoW was in development, quite a number of devs wanted to be able to play as the actual Scourge. As an MMO, I think the Horde (or the "NOT-Alliance" faction, be it Horde or Scourge or Legion or whatever) should have been evil. Over the course of the many years WoW has run, it has become clear that Blizzard just simply isn't interested in telling good guy beastmen stories. We got only a handful of stories, compared to the many, many storylines about the evil conquering Horde. The devs mind as well have committed and have been true to their hearts and just made the 2nd playable faction of WoW straight up evil right from the get go. Other MMOs have playable evil factions (DAOC with the conquering vikings, Warhammer Age of Reckoning with the forces of Chaos, LotRO with the forces of Sauron, etc). No reason why WoW couldn't have had one too.
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Because Alliance leaders have morals and do not go around committing genocide or conquering. The Horde are conquering vikings/mongols who burn, loot, and murder for treasure, glory, and bloodlust.
You can't have a blue-Garrosh. The Horde was culturally predisposed towards conquest, which allowed Garrosh to attain popular support. Peacemakers like Thrall are the exception, not the norm. The Horde political structure is also incredibly fragile and fascist thinking (see the blood oath of the Horde) and allowed Garrosh to ascend and assume totalitarian control with astonishing high rates of obedience, no matter how depraved his orders were. The Alliance has no such predispositions. People like Garithos are the exception, not the norm. Furthermore, the Alliance's political structure has traditions and checks and balances that would prevent such a crazy from attaining power, and the Alliance races value free will and morality and thus would not be inclined to obey unethical orders. Genn becoming High King and ordering that the Alliance start genociding Tuskarr villages and the armies actually doing it would make no sense whatsoever (just as the "purge squad" nonsense in BFA was laughable).
You'll never get an "Alliance centered expansion" the same way the Horde got multiple expansions and games about them. The Alliance is not interesting in of itself. It is interesting compared to what it stands against. The Alliance is at its most interesting when it is weathering the adversity of having their lands being invaded by inhuman forces of evil, their fathers and sons dying, their kingdoms being burned and their heritage lost and humiliated and yet still standing tall and holding fast to their morals and fighting the good fight.
That is what makes the Alliance interesting.
The only vaguely interesting Alliance storyline that doesn't really have much to do with outside forces was the Stonemasons/Defias Brotherhood storyline. Ie, the city was in ruins, stonemasons were paid to rebuild it, they didn't get paid, they rioted and this led to an insurgency/criminal syndicate that thrives in the underdeveloped areas of the kingdom, breeding resentment and gathering up these outcasts. That's actually pretty interesting. Sadly the writers went "it was actually an evil dragon behind it the whole time!" and the storyline was forgotten.
Warcraft 2 had a LotR ending where the surviving orcs fled from civilization. Presumably they dwindled over time and were eventually wiped out.
After Warcraft 2, Metzen came up with the idea for the Lord of the Clans story, so that's when we got the "they weren't wiped out, they were put into concentration camps, also Orcs aren't inherently evil" retcons. That story was fine, but it didn't have legs for what would eventually become WoW. When Thrall's Horde sailed to Kalimdor, they fulfilled their story arc and didn't have any reason to remain in the narrative. TFT's bonus campaign had to bend over backwards to contrive reasons to revive the Horde vs Alliance conflict, and ever since the writers have been constantly tripping over themselves trying to excuse the Horde and preventing the Alliance from just getting rid of these barbaric invaders and serial genociders off of their planet.
You left out the part where the Horde had historically committed multiple genocides and their current ruler (who had popular support and was representative of the will of their people) wanted to wipe out the peoples of the Alliance, and had just wiped out an entire city. These elves (who had sworn an oath of neutrality while in Dalaran) had been discovered to be traitors who were actively aiding the Horde in procuring dark artifacts to wield against the alliance and commit more mass murder. Ofcourse everyone is okay with Jaina acting that way! Also bear in mind that the only elves killed in Dalaran were those who were resisting arrest. She didn't commit genocide.
If there was a demand to play as an evil faction, then Thrall's post WC3 Horde should not have been picked to fulfill that role. This is more ergregious when you consider that WoW was in development at the same time WC3 was in the works, with Metzen being said to have become frustrated with the devs who got the Scourge and Sylvanas' Forsaken mixed up because he had the idea that he was setting up the Forsaken to be playable in WoW. He could have easily written the lore at the time to set up an evil faction with a wide variety of monster races in preparation for WoW (ie, maybe a different Horde altogether, or Scourge with more varied races, etc).
WoW has failed to cultivate new characters and get players invested in them. The only WoW original characters people like on the same level as the WC3 cast are... Garrosh, Varian, Genn, Anduin, and... ??? I'm drawing a blank here.
A personal hobby of mine is browsing and downloading fanart. I have seen too much Warcraft art to recount, and I have nearly a thousand pics of the best art saved in my folder. Almost all is art of the old WC3 characters, + the above aforementioned 3 new characters. And sometimes you might see art for a WoW OG like Wrathion thrown in there now and then, but that's it. There are WoW OGs that have been around for a long time and have a lot of screentime across various questlines, books, short stories, etc, like Velen and Baine, but nobody really cares about them on the same level as the WC3 cast. I don't recall having ever seen fanart for Baine or Mekkatorque and etc. And now that most of the WC3 cast is all gone... Blizzard is out of characters that the audience really cares about. It's no coincidence that since WoD, the character on the box art for every WoW expansion is someone from the WC3 games. Grommash, Illidan, Sylvanas, etc. And our adventuring party in Shadowlands is all old characters everybody liked. At this point, I'm pretty sure Jaina, Thrall, Anduin, etc all have plot armor because if they're gone, then there will be no one left for the audience to care about.
Boggles my mind that he never had those shamans and druids terraform the land surrounding Orgrimmar into fertile fields. Thrall could have also had tried to work out a trade agreement with the Alliance to get Gnomish engineers to build irrigation systems. Would've helped improve relations. The Alliance would have likely agreed as they would have been incentivized to try to turn the Horde into a more peaceful agrarian society.