Having to add females in brink would have cut the over all customization by about 30%...
To add female models, they would have had to cut it form the creative budget... this includes
Animations for EVERYTHING (holding guns, running, jumping, sliding, mantling, shooting, reloading, even breathing and talking)
Animation Smoothing (like moving from one action to the other w/o being jumpy, and looking natural, MUCH harder then you think)
Face Models
Body Models
Clothing Models
Clothing Texture
Skin Textures
Model Rigging
Cross Model Compatibility like animations, rigging, clipping for all clothing styles and options (MASSIVE drain on resources there)
Just ADDING the female body would already be a massive drain. The creative resources in just conceptualizing it and putting a basic, fluidly working model into play would have already been too great of a cost. Let alone trying to get it to work right again across all 3 model types (heavy med light), with no issues or clipping. Then you have to consider each bit of clothing has its own way that its rigged and interacts with the base model animations to look smooth and proper. Again, this has to work across all 3 body types.
Then, having to remodel a new set of clothes would be another massive drain. Its not just "oh take this jacket and throw it on this model" No... It has to be re-concepted to look unique and proper on the female model, specially since SD wouldnt accept just cloning a male part, but it also likely wouldnt work right. New body structure, new rigging, different rig points/animations... would come out horridly wrong. You basically would have to re-create it again anyway...
Sure, you coulda just taken the freakishly proportioned male models, done some re texturing, and gave them some brests... You could get away using the old models/animations then... But believe me it would look just ungodly terrible, and you all would be complaining just as much if not more...
In the end, you wont even have a 50/50 split. The resource in just creating the concept alone would already be detracting from the over all customization that exists. Being that people already think its poor, imagine if you had less than 2/3 of whats in game to work with... All just to have a silly female archetype? No thanks...