Because a ladies' crossbow has to be pink and purple, yes? Because she's a guardian, not a hunter. So she doesn't need to blend in with her surroundings. The Nerf site describes it as a 'fashion bow'. Whatever that means. The boy one* is equally loud, but much less 'fashion'.
Scuse me while I go vomit in the corner.
* Am I wrong in thinking that nerf is like lego, in that really it doesn't need to be gender-segregated, and that doing so is just a bit wrong?