Something to keep in mind with gods and goddesses in fantasy is that often they’re the manifestations of fundamental aspects of reality, meaning their very nature can limit the scope and conditions of their intervention in the material world.
It’s easy to think being a goddess should make her able to do anything lesser beings can do, only better, but that’s not necessarily the case. It’s a common trope in fantasy for deities to be in many ways more limited in their range of interference than non-divine entities because they can’t just arbitrarily expand or redefine what they’re capable of doing, as the scope of their existence causes them to perceive and interact with reality differently than lesser beings do. Especially when they’re abstract and conceptual gods and goddesses of cosmic forces.
In many ways, past a certain point the more powerful an entity is, the less it can do hands-on because everything it does by virtue of its power and scale can have vast and widespread consequences far beyond the singular thing it may be trying to affect. For all we know, as a goddess Elune’s power may always be spread across the entirety of creation as part of the machine of existence, and Tyrande (or anyone else) becoming the Night Warrior may cause Elune’s presence to temporarily lessen or even outright withdraw on a million other worlds until the power can be returned.
Moreover, if we’re to consider the possibility that she’s a out-and-out god, then we have to also consider that she doesn’t adhere to the same scale of moral immediacy that we do. As mortals, not preventing something like Teldrassil may seem like a heinous abdication of justice to us, but the worst things we can imagine happening may well pale in comparison to the worst things imaginable to a nigh-omnipresent deity with an ageless comprehension and awareness of the universe as a whole.
Elune’s also potentially a First One, and in fantasy universes, “creator gods” in particular can often be portrayed as so vast in nature that they literally can’t consciously manipulate events as we perceive them because they outright don’t comprehend existence on the level at which mortals live, so their involvement is more often manifested as roundabout series of events and generalized aligning of circumstances rather than pinpointed cases of tangible intervention.
And atop all that, when one gets into ideas of universal gods and goddesses, one has to consider the possibility that she has been intervening all along, and we just don’t recognize it because the nature of her intervention isn’t usually obvious. Or even that by the nature of such things, the very fact that we and the lore characters act and resist these threats to our world may literally be Elune (and perhaps any other gods/goddesses like her who may exist) making their moves to preserve creation through us. Because if she/they can’t shape the course of events through constantly overt manifestations of their power, they may rely primarily upon nudging and inspiring and influencing those beings who can do so in a billion different ways that aren’t usually perceptible.