Because she’s the hero Canterlot deserves. But not the one it needs right now…
Here’s a pic of Bat-Luna, and before you ask, no I’m not a Batman fan. I was just re-watching the Dark Night, and I realised Luna would make a great Dark Knight. She’s an outcast, even after returning to equestria, and then of course there’s all the questions surrounding her banishment in the first place. Was she banished for a true crime? Or were her actions misjudged? Maybe she and Celestia realised ponykind couldn’t cope with two Alicorn goddesses in their darkest hours, and Luna volunteered to be the fall-pony?