I think it's just because traditionally it's been a male preserve, like in those old westerns with saloons and all male poker players. The only women there where "working". Of course that's been changing for a long time and there are plenty of great women poker players. Still when I play at a local Miami tourney I'm often the only women playing, so I guess some women find that a little intimidating.
I agree, it's the past when it used to be "men's world" anyway. Things are about to change but it takes time. Someone said here that females are not interested in gambling but I call that a bullsh*t. It depends on the person, not the gender. It is just that most people fall (subconciously) into that category where society thinks they should be and what they should do.
It's hard for a woman to even enter the poker world since it is still filled with mostly men and I agree that it might feel intimidating for women. The funny thing is that when I play, there are usually just a couple of other female players but those are the players that I am really afraid of.
So, how to get more female players? Well first of all, everyone needs to stop thinking that females are not into it. Go ahead, teach the game to your girlfriends and ask them to take it seriously while you do that. Let them play and see what happens. Of course there are girls who won't like it but I think you'd be surprised about the amount of girls who actually do.