Orc, human, and night elf forces came together successfully to defeat Archimonde at Mount Hyjal in the Third War. So what happened? Territorial disputes. The night elves were unwilling to offer the orcs any kind of useable land or resources, and the humans were equally unwilling to do so. In order to survive, the Horde had to take the cheapest wow accounts to what it could -- and take what allies it could. Likely the biggest mistake Thrall made in his career as Warchief was allowing the Forsaken to join the Horde. It wasn't something he took lightly -- he had to be talked into it by Hamuul Runetotem, who thought the Forsaken had a chance at redemption.
But that act pretty much closed the door on any hope the Horde had on
successfully coming to any kind of negotiations with the Alliance. The Forsaken
weren't just misunderstood dead guys -- they were former Scourge. They committed
horrific atrocities while they were under the Lich King's sway. What's worse,
after they broke free of the Lich King's hold, they went ahead to sell wow
accounts and began slaughtering the rest of the living in Lordaeron. As far
as the Alliance was concerned, the Forsaken were monsters -- and in a way, they
Between that, and leftover feelings of animosity from the First War, there
was no way any kind of negotiation stood a chance. Thrall's Horde was not the
Horde of old -- but the allies it chose to consort with implied that it was the
same as ever.