Same argument. If she had the power to save Teldrassil, why didn't she always save them? Why let any night elf die, ever?
If she chose not to intervene this time because Ardenweald needed souls, what about all the other times? If Teldrassil was within her power to save, every other massacre would have been trivial to prevent.
Either she can't save everyone, or she doesn't want to save everyone.
And either way, there's nothing new here. This isn't some retroactive change to the story. We were there, we saw her fail to save them. We've seen her fail to prevent massacres of night elves countless times over the last 10,000 years. Rarely does her intervention extend beyond shielding a few intensely devout individuals.
So why are you acting like anything has changed with this scene?
Edit: To put it another way, Elune has always "worked in mysterious ways." She was never reliable. This is like getting mad that God didn't stop [insert recent tragedy] while ignoring the rest of human history.