[throwaway09223]

Because it is not representative of the underlying demographics. There are vastly more men trained to work in the field (one representative datapoint: there's an 80/20 gender difference in computer science degrees).

If you are hiring equal numbers of women and men from a talent pool which is 80% male then you are almost certainly illegally discriminating on the basis of a protected class.

[Spivak]

Or just heavily recruiting from women dominated spaces.

Also say what you will about it, once a company/team gets known for having women other women will flock to it.

Edit: haha people downvoting for the reality that women like working with other women.