I work in 3D Design, I don't have to work for Square Enix to understand the workflow.
Sadly it's not as simple as "Pull these polygons to make boobs!"
Even just to get these models into ARR took a ton of work to "Import" it essentially, the work-flow I can assure you isn't as simple as "drag and drop" a lot of remodelling, texturing and rigging takes place.
For all of the models in XIII and ARR, they work from a High-detailed model then apply those details to a lower detailed model, to make large topological adjustments you would need to go back to the High Poly model and make your adjustments from there which would be no easy feat.
For Snows outfit adjusting it to fit a female shape would NOT be an easy task. The overall body shape of a woman is different, including but not exclusive to the shape of the shoulders, waist, legs and calves. This would be a pretty large task for the XIII and ARR development team to adjust the model to suit this, more so than it's worth in time.
Consider the work load;
Male: Hyur: Midlander, Highlander | Miqo'te | Roedayn | Elezen | Lalafell
Female: Hyur: Midlander, Highlander | Miqo'te | Roedayn | Elezen | Lalafell
That's 12 model adjustments, to make the other counterparts would be a total of 24 adjustments ontop of the model redesign for snows outfit.
This is a lot of work for a quick promotional event, which probably took a month or two to even finish what they had done.
A female model would in some cases essentially be working from scratch, adjusting the model without the original high-poly version would make it VERY difficult to adjust.
TL;DR
You don't need to work for SE to understand the workflow.
Snow is male, to make his outfit work on a female body would take a lot of work, there are a huge number of difference in the shape for men and women that would require adjusting and this would eat up a lot more time than it'd be worth.