I think there are a few reasons why women don't ask for comments. Firstly anyone posting on a site like this obviously has a streak of exhibitionism and likes their photos being looked at, but women don't need to ask for comments, men leave them and that's very nice of them, but men who also have that streak of exhibitionism can go a little short on comments especially from women. Sometimes I'm a little reluctant to leave comments on men's pics because they occasionally see them not as a comment, but as tantamount to us saying 'I'm gagging to ride your big fat cock, please please send me multiple pm's.'
Not all the women who post photos are full of confidence, some of us are but some of us post them to boost flagging self esteem, if we're full of confidence we don't really need the compliments, if the case is that a woman was low on self esteem she probably wouldn't like to take the risk of getting negative comments.
Occasionally a woman will receive a negative comment and even the most confident of us can find this a bit of a knock, I don't view negative comments as such (I haven't had them on this site but I have on another) I view them as sour grapes. Usually a negative comment comes from someone who hasn't posted pics and doesn't have the nerve or body to do so, those of us who do post pics prefer to chat to guys who've taken the same step we have so the only way they can get any sort of reaction from us is by insulting us.
I think it's widely perceived that women are insecure about their body image but I think it's the reverse, on the whole we're much more secure about it than men, you can basically destroy a man's ego by telling him he has a small cock, but I think women put less store in just their physical appearance so they feel less need to ask for comments to confirm what they believe.