Maybe it is because women are also usually responsible for the child care, cooking and cleaning....doesn't leave much time for riding to work.

My Aunt-in law has three kids. One is in college, but she does all the cleaning and cooking, works a full time job, goes to school at night, and takes the kids to their various sporting practices and events. Now when the heck is she going to fit in a bike commute? She is hardly alone. Her husband...a very nice man, owns his own business and mows the lawn on weekends.