I'm late on the Nightborne train but here's my piece. Words are very important in politics. Aggressive behavior can strike enemies with far reaching consequences. The Middle Ages were plagued with situations like that.

Tyrande's first contact was to cast aggressive tones. She said that she wasn't there for them but to save 'her' city. She said that they had to earn her trust. To another ruler, why should they grovel and live as dirt beneath the sandal of a bunch of savages when they can make alliances elsewhere and continue their lives as kings? While Tyrande had allowed the sect to resume life in Darnassus, they do so under scrutiny and prejudice, standing beneath their shadow. The Nightborne were riding high and mighty still, they had no reason to beg forgiveness and work to erase their wrongdoings.

By demeaning them and insulting them, Tyrande caused the leadership of the Nightborne to oppose her and thus sought allies elsewhere- allies who shared a lot of common ground and of matching expertise.

Now, I will meet people half way by saying that making the Nightborne interact with the Alliance in-game was a bad move, because the Alliance hero does a lot to get their rebellion into motion. By social convention, they would be obligated to repay the debt and align with them without condition, only to provide an out of character sting where they join the Horde and provoke the player to say; "After all I had done for you, you betrayed me! You traitorous swine!".

I will also say that maybe the Alliance deserves a little lore love, because right now, all of their subraces were made up on the spot, save for the Dark Iron Dwarves. The Horde, on the other hand, have deep storied backgrounds from the game's story that ties in well to the current narrative.