(Commentary): This is subjective. The last we saw of the Blood Elves, Kael’thas had been scorned by a human controlled by a Dreadlord, failed by Illidan who couldn’t slay Arthas and avenge the Blood Elves, and really, the only positive interaction Kael’thas had with any races that were playable in WoW at that time, was with the Night Elves who… were Alliance in WoW.
(Commentary): That’s where, in my opinion, it becomes evident that Blood Elves were put into the Horde explicitly for reasons not in the story, but from marketing polls. Relations between Night Elves and Blood Elves had actually ended on a pretty good note. They’d worked together, provided mutual aid and support, and Kael’thas even helped save Tyrande’s life. The Night Elves would’ve already encountered High Elves among Jaina’s forces back on Kalimdor as well. It seemed as if the two races were on good terms.
(Speculative): The most organic story at that point would’ve been Kael’thas seeing how Illidan, Vashj, etc… failed. The old ways, the ways his people clung to, of seeking more and more power, simply did not work. Reaching out to the Night Elves, to try another way to achieve survival, would’ve been logical.
(Commentary): Instead, the Night Elves were written to suddenly be antagonists to the Blood Elves. Rather than the cordial cooperation and interactions we saw in the past, suddenly the Night Elves have fielded an army in Quel’Thalas to investigate, ‘reckless arcane magic use,’ there after ignoring it for 10,000 years in Dire Maul. I mean, throughout the entirety of Vanilla the Sentinel Armies couldn’t be bothered to, you know… do their duty and protect the sacred forests of the Night Elves, but suddenly the Blood Elves are going Horde and there’s this thread between the two races that needs to be severed despite it making no sense for the Night Elves to take any interest in Quel’Thalas by their own initiative.
(Commentary): One can argue that Garithos soured the Blood Elves’ relations with humans, but what about dwarves? The Wildhammers in particular have held historically good ties with the High Elves. Between them and the Night Elves there really should’ve been more reason to seek the Alliance. What did the Horde have? The Forsaken? The undead who literally cannibalized their loves ones and destroyed their kingdom? The mere sight of the Forsaken should’ve been enough to get any Blood Elf to pick up a sword and start swinging it. Instead in what can only be the most bizarre twist on Stockholm Syndrome the Blood Elves embrace the Forsaken because Sylvanas is among them.
(Commentary): The Horde really had nothing so vital to the Blood Elves that they should’ve wanted to join it. So many races in the Horde have been traditional and even ancestral enemies of Quel’Thalas. Blizzard had to tailor the Blood Elf starting quests in such a manner as to cut as many of the lingering strings Blood Elves had to the Alliance as possible, and in the process damaged the narrative of the races involved. I already noted how utterly absurd it was for Night Elves to be in Quel’Thalas while their own sacred forests are lousy with Satyrs, Corrupted Furbolg, Demons, Naga, and Orcs.
(Conclusion): Personally I think what is done, is done. As flimsy and damaging at the Blood Elf starting experience is, it is canon. I think it could’ve been done better, much better, but there are limitations to the story that can be told in an MMO. What I find more disturbing is Blizzard’s inability to write the Blood Elves independently of the Alliance. Whenever they come up, they’re always interacting more with the Alliance than the Horde. Even in the case of the Nightborne it feels like someone at Blizzard went, “Hey, I just played WC3 again and Night Elves and Blood Elves being friends seems cool. Let’s make some Night Elf offshoot race for them to be friends with. It just felt so natural and organic and would be great story.”