Attempting to take the empiricist point of view, I think that one has to break down the components and accurately describe the RPG experience, its people, and the personalities that enjoy it. Having said my disclaimer about how there are exceptions and not to get carried away with the generalities (since we deal with majorities that need to be accurately measured to maximize informed context).
I would say that the hobby has traits like sedentary nature, limited socializations, strong familiarity influence (comfort zones etc.), very personal and subjective and have a limited set expressions because of its deeply personal nature. Not really a hobby that a lot of people would take.
Its a hobby that people take really personally so there is very little accommodation to strangers. An outside perspective would find this game difficult to take, especially when the peculiarities differ from their own view of the world. And by nature, women are more inclined to be cautious and men more risk taking (i'm speaking of generalities is behavior). So I think this cautious nature, which is reasonable in my understanding, is a very valid reason why not get tangled up in the deeply personal ideas of the gamers.
Also if we take into account the gamers who lack any socialization and their personal ideas regarding the gender will have a very damaging effect on female participation. Just one gamer with distasteful views can turn away a much larger number of women gamers, while the most charming of gamers are not in the job of recruiting the most difficult to convert (because in the time vs output its the least rewarding).
Joethelawyer's Wondrous Imaginings has the list of all the other points