Nightmare Moon has spent years of accumulating enough magic for herself to materialize after her defeat by the Element Bearers. She is separate from Luna. Her own being. She does not want eternal night anymore, having realized it would doom Equestria.
She can only think of the Sun. Of how dear is the Sun for her actually. She wouldn't take it from the skies... Everything needs her.
Even dark creatures as herself.

Cover from 90sigma's DeviantART

EDIT: Wow. Featured. I didn't expect that. You guys are wonderful. There will DEFINITELY be a sequel to this now.