I bet the hangup was the assumption that "if we add them they have to look really different or you won't be able to tell the difference at a distance!" Making women and men look easily distinguishable underneath all that gear probably couldn't be done fairly in terms of gameplay (because of visibility and shootable size.) It's a variant on a classic sexist catch-22: women are a threat if they're different from men and superfluous if they're the same.
Of course there's no need for men and women to look obviously different from forty feet away, but in the past, it might have been a bigger deal. I remember that when I was a kid, it was a huge thing when a man mistook a woman for a man or, much worse, a man for a woman. If a man mistook a long-haired man for a woman, even from a hundred feet away, he would be the subject of derision from his friends, and he would be panicked and furious. The mistake would be blamed on the questionable sexuality of one or both parties. From where I'm sitting it looks like there's been a massive cultural shift in that regard, and I don't think anybody will be playing CoD filled with homophobic terror at the thought of mistaking a character's sex. It might have been a concern for game designers in the past.
Besides sexism on the part of the game designers, there's also the fact that the U.S. armed forces very recently decided to officially allow women into combat roles. CoD will have the game out before the new policy fully takes effect.