Obviously they want "Mandalorian" in the title so people casually familiar with the show know this is a movie for it, but want it different enough that it doesn't cause confusion about what people are talking about. Personally, having not seen it of course, I'd have probably gone with "Star Wars: The Mandalorians", since there will definitely be more than one Mandalorian in it (and the last season of the show really became as much the story of a Mandalorian tribe as of the Mandalorian), and since whether or not Grogu becomes a Mandalorian will definitely be a major character point. Alternatively I'd go with "Way of the Mandalorian" as that both alludes to their catch phrase and makes it sound like an old martial arts movie, it's own form of pulpy goodness.