To expand on Chronorabbit’s answer to this.
A faction rivalry in an MMO draws on an eclectic brew of psychological drivers from the simple conditions of a rivalry itself to the personal investment in an RPG, to the core reasons for why we play video games in the first place - those being to fill psychological needs for autonomy, competence and relatedness (that is that we can make decisions, that we believe we can effect our decisions, and that our decisions matter to the world around us).
To summarize that, as Neo put it: the problem is choice.
We choose our avatar. We experience the world through their eyes - we step into their shoes. Our behaviors actually change subtly to fit the character that we are playing (a thing observed as the “Mimesis Effect”) We are led to care about the things that they care about, and they are in our minds the protagonist of an inherently emergently told narrative about the things they did in traveling the world and taking part in these adventures.
Through Vanilla and BC, while yes, there was a story in the background, most of this experience was relatively freeform and emergent. Our tale was consistent of our actions - the quests that we took, the things that we did, the friendships that we made. But sometime after that, Blizzard decided that they wanted a more active role in storytelling - they wanted big, flashy epic things about their characters, which takes a lot of our choice away. This can be annoying - having all of the important things done by the important characters - but it’s not actively toxic.
It got that way when the faction rivalry was run through this mindset.
Dave Kosak described the framework as “the Hero Factory” in response to accusations of faction favoritism in 2011 - that heroes only rise through adversity, and therefore really bad and sadistic things had to happen in order to encourage heroes to rise. This was in response to widespread Alliance criticism that Blizzard had, by fiat, established that the Horde was beating the tar out of them all over Azeroth.
Now pause and recognize what that is - this is stepping into a competitive, multiplayer environment, and - instead of relying on the illusion of choice and competence to make people feel, through their gameplay, that they had either accomplished something or needed to work harder - having the developer put their thumb on the scale and DECIDE who gets the awesome moments and who gets to look like a loser on the basis of nothing more than their personal preferences. I’m also confident in saying that because the Alliance as an entity typically doesn’t have its victories portrayed, or it’s made to look weak, or the victories aren’t said to matter - and crucially, the devs put little to no attention into the critical task of making the Alliance at least appear to be just as awesome and just as interesting as the Horde - they were just bored with the whole idea.
The Horde meanwhile is split. You have people who are into the rivalry who enjoy winning, regardless of that feeling being manufactured, who want to keep winning. You also have the people whose entire faction identity is getting trampled because of the Hero factory concept - and it was the exact same problem in Cataclysm as it was in BFA - only more intense. The psychology of the rivalry means that neither side will give an inch - and we have seen the results of that as Alliance brand equity continues to slip, resulting in more and more faction balance issues both here and in Classic. The Alliance is boring and it sucks. The Alliance is cringe. The interesting races of the Alliance were sanded down, and once again, no one wants to fix it.
What is the most important thing in an RPG? It’s your role. It’s your identity. What has Blizzard done with respect to the faction conflict to that identity? They have attacked it, viciously, and they took away your agency in what you can choose to be, how competent you are, and whether your actions matter or not.
You aren’t just going down a bad road, you’re powerless to stop it, and the conflict with the other side is to blame. You have bad emotions that a competently run rivalry would allow you to work out through gameplay. This one, if you’re on the Alliance, repeatedly denies you the catharsis that “the Hero factory” implicitly promises - as it did in this case, and that feels awful. If you’re on the Horde, you’re told that you’re pure evil when you may not have done anything to deserve that. Your choice is thrown to the wind, and people like me will be here to deconstruct whether your motives were ever really good in the first place.
That’s the closest I can get to an answer for you. If you want a summary - it’s because Blizzard crafted a scenario that would generate this kind of hate with an artisanal precision.
