They had him write it because he understands the character and would know how the character would logically fit in on his own, right? I understand how tying Rogue into the movie would work like they're doing Colossus for Deadpool, but it could also make a mess of things too, right? Plus, in Gambit's origin, Rogue isn't really present at all. It wouldn't make sense for her to be down in New Orleans during the Assassins Guild story whereas other elements like the Marauders and Sinister could fit in more easily, being an ex machina to start the story.
Also, she wasn't in the original Assassins Guild story so for those saying Gambit and Rogue should be together in the movie because of the comics. Well... if it is Assassins Guild, which makes the most sense, then if you want to follow the comics, no Rogue.