I've asked this question myself before and have come to my own conclusion.
Ego. Or perhaps politeness.
Unfortunately I suspect this is a place where male ego actually works to a males benefit.
Women are generally very compliant to rules and respect both boundary's and protocols in life.
Men seemed to be brought up NOT to conform, to break rules and show that they can make their own ...as if non conformity is a mark of manliness.
Transfer that onto the road and women may stick to the "correct" position on the road even if it is hazardous..where as a man wont worry about darting about and ignoring conformity.
An everyday example...on a country road i will take the adverse bend to see right around the corner, placing my car on the wrong side of a white line to do so. I know a few lady drivers who will never do this and are constantly on the brakes as oncoming cars "surprise"them when they appear around a bend.
I suspect the desire not to look like a flash git or a show off...in other words to ride politely may be a cause
Also, i think men are brought up to see driving as manly skill and so tend to drive more than women (my wife's dislikes driving anywhere she doesn't have to and would never just go for a drive for fun) as a result more men are likely to have experience of driving in town and translate that road knowledge to the bike. I see a lot more men driving in town than women.