I've got to answer my own question. The numbers-- how many women lawyers relative to total lawyers -- mean nothing. Gender discrimination isn't holding women back. Women earn as much as men do in most major cities, maybe even more. The inequality between men and women in the legal profession, in the workplace generally, is not gender but rather children.
Two person partnerships like marriage are prone to deadlock. Recurring negotiation over division of labor is exhausting, destablizing and inefficient. So, marital partners specialize. Among the range of activities partners undertake, childrearing particularly demands specialization. It begins with biological imperative. It continues until the child is an adult. One parent becomes the principal source of care for the child when he is helpless, sick, learning to read, or needs a ride to the orthodontist. One parent knows the names of all the child's toys and all the words to "Wheels on the Bus." The other parent, if there is one in the picture, becomes the surety parent, the backup who steps in as a substitute for the principal when circumstances require. Said another way, the principal parent is "on call." The backup parent is by appointment only.
The family captures the benefit of specialization. But, the principal parent loses ground in the workplace. This parent foregoes advancement in a job market that understandably values "on call" workers and devalues workers who are already "on call" for their families. At least so far, women choose or otherwise find themselves in the role of principal parent more than men do.
If a man and a woman start out as equals, why does one of them forego career advancement in favor of specialization in family? Some blame men. Others call it a mystery.
I think it's love.