https://www.wowhead.com/news=291269/
这篇第三个方格,吉恩跟珍娜的对谈中可以看出原因
Jaina: Sylvanas ordered the execution of Baine Bloodhoof. Anduin sent us into
Orgrimmar to rescue him.
Genn: You risked your lives to save a leader of the Horde?!
Jaina: After what he did for Derek, I felt I owed him a debt. We received
some unexpected help from Saurfang... and Thrall.
Shandris: You were fortunate that you were not betrayed. Presenting your
heads might have regained their warchief's favor.
Jaina: We had a common enemy. Sylvanas wanted us at each other's throats.
Perhaps that's what Azshara wants, too.
简单来说就是还贝恩一个人情,外加艾萨拉女王威胁在外。
这时候先共同对抗最大的威胁显然是个比较明智的方案。