Women have on average shorter torsos, therefore a shorter reach compared to men at the same height.
Their shoulders are not as wide as most men of similar height, so a wider bar may not be as favorable.
Quicker steering might allow for a lighter touch (although I'd think you'd want slower/less responsive steering).
Women are shorter in general, so having a step below a Small might be necessary.
Thing is though is that modern long reach long front center unisex designs with lower seat tubes and better standover would actually provide many women with the ability to rock a 35mm stem and still fit comfortably on a "mens" bike.
Rather than doing completely separate lineups I just wish they made more bikes with lower standover and perhaps a WSD build kit (different seat, bars, stem, brakes with shorter reach, etc.)