Garrosh at that point wanted Ashenvele and probably Azhara for the Horde. I dont know about you, but I very very very very highly doubt the Night Elves will ever willingly give that up, on the battlefield or on diplomatic talks.
I mean at best what, the Horde would get a few miles on the border? That hardly satisfies the future he wants for the Horde.
Plus, Garrosh sees the Alliance as a great enemy of the Horde, and enemies exist to e defeated, not coddled with.