Because that type of female is overused.
Guns are over-used. However, they work pretty well. Why bother with sharp sticks?
I understand, variation is good. I do agree. But I personally expect consistency, too. A great game with great mechanics, great story, great this and great that shouldn't limit itself to sub-par character models. I know, we can't exactly play as Brad Pitt and Johnny Depp, but we can lighten that blow with masks and jacket hoods. If they left us with only lumberjacks to use for female characters, I can say I'd be very disappointed. To keep up the Brink art style, yea, a gal's facial features would be exaggerated. Not to be insensitive, but we could surely mask/hood the females, too. I expect the character models to more focus on body shape that sixy faces. Considering they would need a body shape for lights, I don't see why they'd decide against slim, attractive build because it's too ubiquitous. As I stated, they've essentially locked themselves into that slim build requirement by offering the light frame character.
Essentially, if they add female characters for player use, they're not gonna make the light female stocky because of how frequently every medium sells attractive figures.