I agree with what Wild@heart wrote. I didn't get a cdl until I was 50 years old even though all my life I had been fascinated by big trucks. I worried that being a petite female I would not physically be able to do the job. I even worried about being able to see over the dash and reach the foot pedals, lol. From what I have read on message boards, many women also worry that they are not physically capable of being a truck driver.
Another reason, in my opinion, is that even today women are expected to be nurturers and caregivers. For example, when it comes to going on the road for weeks at a time, it is usually the man in the family who will go and the woman will be expected to keep the home fires burning, taking care of the kids and the home. Not always, but more the norm than the other way around. I think it takes a woman being a bit of a rebel, to buck the norm and be different, to go into trucking
Also, I think many women are afraid, being on their own in strange places with mostly men. I've seen a lot of women asking questions on message boards about safety issues in rest areas and truck stops. Women who have grown up with truckers in their family are usually more informed and know that, for the most part, they would be more safe parked at a truck stop than they would be in a Walmart parking lot doing their shopping.
I think it takes a special kinda gal to be drawn to trucking, one who is not only a rebel but a tomboy and a daredevil, someone who feels comfortable with the guys and doesn't mind getting a little grease on them or squatting behind a bush now and then, someone ready to accept a challenge and wants something different. I also think it takes a special kinda man to go into trucking but I believe those men are drawn into it for different reasons, maybe loners or a sense of freedom. Of course there are many men who are drawn to driving simply because they need a job and end up hating every minute of it, while women needing work usually migrate towards traditional "womens work" type jobs, mostly, I think, due to society's expectations for gender roles.
So, in my opinion, fear, society's expectations, and lack of education and self confidence, are the main reasons more women aren't trucking. Also, targeting advertising towards recruiting a certain personality group might attract more female drivers, just as advertising "freedom of the open road" seems to attract a certain group of men