They did supply the explanation…“We are abandoned, forsaken…etc.” For whatever reason, you cannot connect the dots. Movies have ratings because children may not fully understand the content they are watching. Your comments would suggest a similar inability to grasp certain elements, beyond a one-dimensional narrative.
If you pay very close attention to Sira’s dialogue at the beginning of the Tyrande Ascension scenario, you’ll find that it’s not realy that different than the message she puts out as a Dark Ranger. Even while alive, aiding Tyrande, it’s clear that she had already rejected Elune in her heart, and her thought was widely divergent from both Tyrande’s and Maiev’s.