Because Powell used mandamus. Mandamus is only granted when all other remedies are exhausted. She hadn't used up all possible remedies so the en banc panel didn't grant her appeal and sent it back to Sullivan.
One apparent remedy that she apparently didn't try is this:
This plea appears to give the decision to dismiss to Sullivan. If Sullivan denies that plea and there are no other options then Powell can try mandamus.
I don’t give a hoot in hell about this procedural nonsense.
The justice system’s first duty is to truth. It supersedes rules of criminal procedure and anyone who thinks otherwise is a traitor.