Whenever the Horde was under the control of a power mad dictator like Garrosh or Sylvanas and they were in trouble, they turned to the Alliance for help. Unlike the rest of the Horde, they’ve always kept a largely friendly relationship with their Alliance counterparts. When Baine was rescued, he came to Stormwind for sanctuary as did Mayla Highmountain.
Despite their tribalistic nature, they have always had more in common with the Alliance than the Horde in that they are largely peaceful and don’t seek out conflict which constantly puts them at odds with their allies in the Horde that are of a significantly more warlike bent. They’ve historically maintained such good relations with the Night Elves that they even consented to train them as fellow Druids who largely considered them peers instead of mere students.
The Grimtotem might fit in with the Horde’s ideals, but the rest of the Tauren? Not so much.
And even under wise leaders like Thrall, questionable practices went on within the Horde particularly with the Forsaken that the Tauren found extremely distasteful and had difficulty reconciling.
They’ve constantly questioned their relationship with the Horde which is only fair considering it has dragged them into entirely needless conflicts with the Alliance not once but twice now. So why do they realistically stay with the Horde when outside of a few instances it has largely been a negative experience for them?
Some in the Alliance might hate the Horde, but I very much doubt there are many who have a legitimate grievance they can lay at the feet of the Tauren people. Heck, even Genn Greymane and Jaina Proudmoore have a lot of respect for the Tauren whom they see as fundamentally ‘different’ from the rest of the Horde. As an Alliance player, I can relate to the Tauren as a people in a way that is almost impossible for the rest of the Horde.
When the Tauren needed them, the Alliance always came to their aid. They’ve traditionally been better friends than they’ve been enemies, so wouldn’t they make even better allies?
While the Tauren have been essential as a voice of reason and morality for the rest of the Horde, frankly it’s time the rest of them grew up. The Tauren don’t need to be the Horde’s emotional crutch nor should they feel obligated to be. If they can’t survive without the Tauren being their indispensable morale compass, do they really deserve to?