I believe that there is some sexism from the adolescents and young adults, but that's not the reason. I believe that it's very difficult for a woman to be comfortable with a hobby where all the participants are male. Rolling dice, shouting, and killing things tend not to impress anyone, particularly if you feel alone and out of place. In Vampire live- roleplay I would estimate that there is a slight majority of female players, these recruit their friends and other female players replace dropouts, keeping the numbers high. In most games, the few female gamers will probably have a tough time convincing their friends and when they do the all-male preserve is a tough obstacle.
Many people get on their high horses to talk about 'great causes'.
I believe that for many gamers when they pick up that dice, their last remaining brain-cell goes on hold. Before you hack a sword into someone just for a laugh, or do a cretinous action have a thought for the poor character you are playing and for the referee who has gone to the trouble of preparing the game. I have loads of gripes, I especially hate the venue which is too dispersed. An attempt was made to centralise the event, but it was worse than last year. I couldn't console myself drinking myself silly.