Because he came to the Alliance trying to suck Varian's dick after this quest:
What's in the Box? perhaps?
Where neither Aethas nor Rommath actually died. And both were aware of what was inside. Rommath even compared it to trapped demon or spirit. And it was Rommath who urged you to open it (Aethas was somewhat unwilling).
This is how much Horde matters to Lor'themar. Less than two NOT dead Blood Elves. He is even more of a nationalist/racist bigot than Garrosh, the only difference is that Garrosh actually has the balls to be open about his bigotry.
Keep in mind Wrathion doesn't have details about what happened in Orgrimmar. When you talk to him when he throws a furious fit in the tavern he says none of his agents could give him a clear picture and during his final audio files he inquires the player as well. Perhaps the identity of the new Warchief is one of the details that did not reach him.
You mean the Sunwalker Dezco that was not ordered to go into Pandaria with Nazgrim's expedition and went there on his own disregarding the fact he was a soldier of the Horde?
The Sunwalker Dezco who, when inquired by the player in Krasarang (which was the only member of official Horde expedition or Nazgrim's underling in that zone) about re-capturing Anduin (which was the official Horde expedition's directive) laughed it off? Only because teenage humans apparently give him a major erection?
Or the Sunwalker Dezco who goes to battle carrying babies and is potentially the one who then crashed one of them to death?
Perfect Warchief material here. Not only does he have no regard for orders or military hierarchy and lets his human perversion cloud his judgement, he can't even keep his children safe. Let alone a major faction.
You don't really want any black dragons taking a deep breath.