The enemy of my enemy is my friend, right? By that reasoning, Alterac was an actual former Horde ally in the Second War, their assistance was invaluable during their conflict. As there was literally no bad blood between the two sides, I'm not sure why the Syndicate is even hostile to Horde players. And the Defias seeks to destroy Stormwind, the Alliance's strongest kingdom -- and they hated Varian and his monarchy as much as most of the Horde did. If you remember the Deadmines on the Horde side, the only problem the Horde had with Vanessa's plan to bomb Stormwind was that her plan was poorly devised and carried out against the Alliance, not that they disagreed with her.
https://wowpedia.fandom.com/wiki/The..._Kingpin#HordeThe Defias plan to finish their Juggernaut and use it to lay waste to Stormwind. It is a noble cause any true orc warrior would embrace without a single thought!
Vanessa's fault lies in her execution of their plan. The ship is poorly constructed, the crew practically untrained, and, worst of all, there's a worgen in their midst!
His name is Admiral Ripsnarl, and he must be mercilessly slain in the name of the Warchief. Lok'tar ogar! It's a shame that he was both a human and a worgen. Had he been a proud orc warrior I'd have liked to charge into battle alongside such a fierce combatant. We can speak of ill-fated desires another day. It is time to take what is ours from the enemy.
The Shattered Hand has laid mostly quiet for years now. No more! You will be the instrument of our rebirth, <name>. Slay the Defias Kingpin and commandeer her ship for the Horde! We will outfit it as a proper orc warship and use it to finally lay waste to Stormwind
Any survivors will spread word of our unstoppable power across all of Azeroth, and all will tremble before our might! Go now, VanCleef's death will be your greatest triumph yet hero.
COWARD!!! Vanessa VanCleef is a dirty human coward! How dare she flee from battle against the Horde and leave her unworthy subordinate to fight!
<Kagtha composes herself.>
No matter. Her ship is reward enough. The citizens of Stormwind will bear first hand witness to our awe inspiring power using a ship made in part by the very architect of their city!
I mean, literally an entire race of Lordaeron humans are loyal to the Horde, and their princess Calia and Derek Proudmoore, Crown Prince of Kul Tiras also, and the Fogtail Freebooters helped fight for the Horde against their own people in the Fourth War:
https://wowpedia.fandom.com/wiki/Fogsail_Freebooters
So why don't the Syndicate remnants of Alterac, together with Vanessa and the Defias Brotherhood try to ally with the Horde? If you remember the dungeon, the trolls are the ONLY Horde race absent, they apparently now accept members from all the other core races of the Horde, so there are no racial hostilities preventing their Horde membership. I mean, don't they still have genuine and lasting grievances against the Alliance, especially Stormwind, for destroying their peoples and livelihoods? Remember how the blood elves fairly easily turned against the Alliance, even after Quel'Thalas was allied with the humans for thousands of years?
From Alterac's perspective, they merely wanted to survive, to protect their homes and families, against overwhelming and uncertain odds -- even in our world, I can envision some governments aligning themselves with alien invaders if they were to attack this planet tomorrow, to ensure their survival. Perhaps they believed that some humans surviving was better than none. And perhaps their relations with the other kingdoms even before the invasion were never that friendly -- the Alliance probably killed more of their population ultimately than the orcs did, ironically enough. I do not necessarily agree with their betrayal, but I think it is arguably understandable, the Alliance only very barely managed to triumph against the Horde, and that was due to their internal fracturing rather than any real military advantage. And from the Defias, one need point only to the elitist and corrupt House of Nobles appointing Turalyon as a sign of their lingering immense influence in Alliance politics, perhaps Turalyon is listening daily to their counsel even now. There was no attempt from the Alliance governments to assist the ordinary working-class people who became hardened Defias members during the Cataclysm, nor did Varian seem to ever truly address their legitimate grievances.
So why can't Alterac / the Syndicate and the Defias under Vaneesa theoretically form an alliance of convenience with the Horde?
P.S. In the interests of faction fairness, of course, if Alterac / Defias humans are made playable eventually, or even if they just quietly join the Horde in the background, the Alliance would also likely receive something in return. But at this moment, I'm just not sure what exactly.