But she can’t tell you why she chose. Literally all she says is “Because reasons.” Which means that Blizzard doesn’t have a reason.
I think it’s less a “For the Horde” thing and more of a “Elune failed me” thing.
There’s nothing to indicate that the worship of Elune is based on transactional piety.

That’s basically it, amusingly enough. When you start the scenario, when one of the night elves makes a usual Elune watch over us or something, Sira is very quick to jump in and say “Where was Elune when this whole mess started?”
I’ll post this again, since you apparently missed it:
Do you see the encampment on the beach below? The Horde followed Tyrande here. No doubt some of them have remained here in the shadows in case she returned. Kill them.
But everyone hates Sylvanas. And the undead. So mind control is the only way.
I don’t care much about Sylive, to be frank. I’m just more amazed that Blizzard can serve up this tripe and paper-thin justifications and have people devour it.