Two key reasons I'm sure are statistically significant, if not the prevalent factors, are added health care costs and lack of productivity related to pregnancy and remaining discrimination against women in the highest pay bracket. This includes executives, doctors, dentists and the like - all of these are characteristically skewed much more so than lower or middle income fields like teaching, manufacturing and retail.
Arguably a woman working at Wal-Mart earns the same as a man.
Women in their 30's are all but expected to have kids, and though there may be no hard evidence of it, there seem to be "waves" or clusters of pregnancy in offices. Perhaps there is a biological or neurological factor related to the well known (and well supported) synchronous menstruation phenomenon. If this is not scientifically verifiable, it is at least a common enough myth that it definitely impacts hiring and promotion in the workplace.
You're going to tend to promote men because they aren't going to bail on you for six months and cost you countless hours in productivity due to complications due to pregnancy. Thus women plateau professionally much sooner than men.
Your medical fields are well regulated by private organizations like the AMA and the ADA, which keep competition low, salaries extremely high and arguably, too, ensure that only the highest quality of care is provided to the patient (that's their sales pitch anyway.)
Anywho, there are negatives and positives to tight regulation by the AMA or ADA, mainly your Dentist or Doctor cannot wipe his/her butt and the turn around and work on you without gloves. This is good. Unfortunately health care costs are through the roof, and these organizations tend to reflect (intentionally or not) a bygone era of what we might call sexist/racist policy. Back then it was only watching out for the "good old boys," but the good old boy's club tends primarily to include only the white, Asian or Middle Eastern male with blacks coming in a distant third or fourth, hispanics following far behind, and women being included as more of an exception than a rule.
In other words they are dinosaurs. Many are truly passionate, caring, intelligent caregivers who have devoted their lives to an altruistic field, but many are also approaching 80, so their points of view date quite literally to the fifties.
The Surgeon General and other such regulatory personnel are not spring chickens as a general rule. Progressive thinking is a virtue of the young, while experienced conservatism tends to be a function of age. In medicine, conservatism is not necessarily bad, but it means social change is quite sluggish in that field.
Disasters like Thalidomide have only served to reinforce such conservatism.
As far as the business world goes, they are in a world of their own. They are unconcerned, largely, with the society at large as they exist in a culture all their own, populated by white men in ties driving golf carts and drinking martinis.
This of course is slow to change, too.