There’s plenty of reasons that have already been discussed on these forums, and throughout the internet. The failure of Saurfang as a convincing figurehead of the rebellion, Baine becoming a traitor and furthering the idea that the Horde and Alliance might not continue.
I think the biggest reason, is Sylvanas. Sure, she wasn’t really present for most of the time. Instead we were working with Nathanos, Rexxar, and Lillian. But ultimately Sylvanas is the one that was always going to have the biggest impact. Her pull on the narrative was so big that it was never going to own up to what should’ve happened with the set up that Battle for Azeroth was given.
Yes, she’s always been dubious at best, and had always had a tinge of darkness. This could be foreseen as early as Cataclysm. Yet, it felt like she was going to lead the Horde into a direction that defied that, that what she was doing had reasoning that made sense. before BFA actually came out, we even saw Sylvanas outright say why she wanted Teldrassil, she internally gave a clear idea why she attacked the undead at the Arathi reunion.
It almost looked like we were going to be given a story that had actual depth, exploring on why the Horde does things that look bad. But then, that cinematic was released. The one, where Teldrassil burned.
I’m not sure if it mattered to anyone else. Hell for many, they might have expected that kind of behavior completely, but for me it was a turning point, the first among a few. It was then I knew we were going to get something not as thought provoking as I thought it would be. I rationalized it as propping my hopes up higher than I should “It’s warcraft, right? It’s never been too deep.”
Anyway, Sylvanas’ intentions were never clear. She was always in the shadows and working behind everyone’s back. It’s likely part of the reason why she was chosen to be warcheif this expansion, for her storyline to be in the spotlight. This was the main problem.
They never addressed it, they never put us into her perspective. In the past she was fully willing to give us an idea of where she’s at, she’s in fact constantly reminding us of that in Silverpine questing. Which was why it was disheartening to see that all along she was just lying the whole time.It’s not surprising, to be sure, but it just doesn’t make sense in my mind. She represented the Forsaken in more than one way. Some people thought this was a bad thing, but I liked that she was essentially the living embodiment of what it means to be undead in a world that rejects you.
Hell, there’s many points where you can say it doesn’t really match up with what was presented before. In before the storm, you can read it to where Sylvanas was trying to keep an upper hand over Anduin to keep the Forsaken, and the Horde safe. Even if she stated her end goal was to destroy Stormwind, this didn’t read as a villainous thought at first. It just read as someone willing to go any lengths to best serve her people, and by extension herself.
I just feel like it could’ve been much more different, and I know I’m not the first to lament this. That maybe if she was more open about why she did what she did, then maybe we’d have a better reason to dislike her, or possibly sympathize with her. Now it just feels like they took a character that had so much potential, and threw it away. Just like Garrosh.
In this way, it is just like the end of mop, and all of the memes about that has a core of truth. It’s technically not exactly beat for beat like the mop storyline, but the end result was the same.
TLDR; They didn’t write include Sylvanas in the campaign or expanded on her motives well enough.