Same reason why the story for Empire Strikes Back came from George Lucas, but the script was written by Lawrence Kasdan?
I would think Moore and Braga had more to do than just pitch the story, though. Even if Robert Towne wrote the script, giving Towne a plot, a framework, characters to use, ideas to process, etc, is all part of the story, something Moore and Braga did
I'm reminded of Yesterday's Enterprise. The story is attributed to two writers (Ganino and Stillwell), but it took four other staffers (Moore and Behr included) to tweak the teleplay. Even though all four staffers are credited, the story is considered the baby and brainchild of the original two writers, to the point where Stillwell even published a behind the scenes book about the episode as well.