They revealed (via ComicBook.com) that before the movie was known as Rogue One: A Star Wars Story, they considered many alternate titles.
We once did a whole lost of things that appeared in its trailer but didn’t show up in the finished film.
Rogue One famously underwent many changes through the course of production.
So like Star Wars: Rebellion, Star Wars: Rogue One, let’s do a title that’s shorter so that even from the title of the movie you know this is something that doesn’t necessarily conform to the unwritten rules of the Saga films.