That’s what Jaina did.
Circumstances change as well. The Alliance didn’t threaten or even do anything to the Night Elves at Hyjal. Arguably, the Horde did more damage to the Night Elves than Azshara did. Or at least equal.
If the Alliance was that concerned about Azeroth above all else, they wouldn’t use Azerite. But they are.
There is also a fair chance that Sylvanas sent the Horde to Azshara anyways. Why should the Horde be trusted to actually defeat Azshara for the good of Azeroth? And that they don’t just plan on backstabbing the Alliance at the last minute?
Idk how much the Alliance as a whole knows about the Horde rebellion and I bet Tyrande certainly doesn’t because of Saurfang being freed.