Generally the information and research we have on the majority of sexual fetishes involves heterosexual men. Although it is recognised that there are women who have sexual fetishes, it is also thought that men are more likely to have a sexual fetish than women. If this is true, then it stands to reason that, if the patterns of diaper fetishism are comparable to those of fetishism generally, there will be more men with a fetish for diapers than women.
Of course, not all people who are interested in diapers are interested in the sexual side of things. Surveys on Understanding Infantilism (Girls, Boys, and Diapers) somewhat support the theory that women are less interested in the sexual side - it found that men are more likely to be Diaper Lovers (who are overwhelmingly sexual fetishes) than women, and females are more likely to report being Adult Babies (who are much less likely to report sexual interest) than men. The report linked to above is a very interesting read, and if you are interested in AB/DLs and gender, I would recommend taking a look at it.
Another theory is that men are likely to be more open about their sexual fetishes, whereas women may not be as out-spoken about it. We live in a society where it is far more culturally acceptable for a man to be very out-spoken about the things that turn him on and about his sexual pastimes than it is for women.
As an example, if a man starts a conversation with his friends, "So, I was having a wank last night..." they probably won't think much of it, and will probably laugh or chime in with their own anecdotes surrounding masturbation. If a woman was to talk about masturbating in a group, it is probably more likely she would be met with surprise or horror - it is not as acceptable or normal for women to talk about masturbation, and probably sexual fetishes in general, at least outside of their close group of friends. There is still an expectation that women be "lady-like", polite, reserved, and coy, whereas men can be loud and brash and crude without it damaging (and in fact, often with it building on) their reputation.
Furthermore, women are far more likely to be sexualised and seen as objects than men, and so may have reservations when it comes to being open about sexual fetishes outside of their very close friends/partners for fears that men will begin to see them as easy or as some sort of prize to be obtained. For these reasons it may be that men are more likely to share and be open about their fetish or interest in diapers than women are within the AB/DL community.
On the other hand, some suggest that it is the AB/DL community itself which puts women off becoming active or introducing themselves. The AB/DL community is a very, very male-dominated place. AB/DL women are almost seen as myths in many places of the community, and a huge number of AB/DL men dream of seeking a girl who shares their interest. This can seem very intimidating. The vast majority of AB/DL websites closely resemble porn sites, and focus largely on the fetish side of this interest. If Understanding Infantilism's report is accurate, and men are more likely to be interested in diapers as a sexual object, and women less likely, then again this could be another intimidating or off-putting aspect of the community.
I can say from personal experience that the above is true for me. Diapers are not a sexual object for me, but in the AB/DL community I am surrounded by websites with professional pictures of AB/DL women in semi-pornographic poses and men talking about their sexual attraction to girls in diapers. As a result, there are plenty of men out there who are interested in talking to me, a real AB female, essentially just to get their rocks off. They want to talk dirty in role-play, they ask inappropriate questions or details about what I wear with my diapers or how I change them, they constantly reference sex in any conversation, and a lot of the time it feels as if they are assuming that real life ABs are just like the models and actresses in AB/DL videos/photographs - a naughty adult baby girl just desperate for a man to change her diapers and then use her to expel his sexual urges. As a real person, I am far more interested in talking to other AB/DLs about real life things than I am in engaging in sexual roleplay or answering their creepy questions. I am also not naive enough to think that sharing an interest in diapers is going to make me instantly compatible with any guy I meet on here, but there are a number of men who seem to think that because they are an AB/DL man, and I am an AB girl, eventually I will probably want to meet with them, baby them and then have rampant diaper-sex with them. I won't.
On top of the sheer number of creepy men who want to use you as some sort of sexual object, there are also the men who simply will not believe that you could possibly be a real AB girl. They decide that actually you must be a man pretending to be a girl, and constantly try to out you as a man (hmm, sounds to me like a question a man would be more likely to ask than a woman. Fake!) or constantly demand proof. Usually the proof should be a picture of yourself in nothing more than a diaper. All of this gets pretty wearing after a while, and so you leave the community behind you.
ADISC is a breath of fresh air compared to the majority of websites. It is a place where the fetish aspect is downplayed, members who say they are female are believed, and men are not creepy and talk to you as a real person, rather than as a porn star. However, it took me about nine years of visiting and promptly leaving AB/DL websites to find ADISC. I dare say there are plenty of other women and girls out there who are stuck in the continuous cycle of joining and then leaving creepy or male-orientated websites, or who choose not to make themselves known and instead just lurk the boards so that nobody contacts them. I suspect that there are more female AB/DLs who have not made themselves known percentage-wise in the community than there are males, and that thus there are probably more female AB/DLs in existence than numbers from polls and surveys show.
That said, on ADISC there are over 2100 members, and only 200, less than 10%, are listed as female. It seems unlikely that, if there were an equal number of male and female AB/DLs in the world, 70-80% of the female members that, based on numbers of males, should have joined ADISC haven't. My guess is that there is a higher percentage of female AB/DLs than 10-15% of our total population, who haven't joined for some of the reasons listed above or others, but based on the numbers there are still almost certainly more males than females interested in diapers.
"Why?" is an interesting question. As a female myself I am certainly curious as to why I am in a minority. In truth it is probably for a combination of reasons, probably including and beyond those listed by myself and the members who have already posted, although I will say that from my point of view I don't think the sanitary towel argument is a valid one, or would be relevant for most women who potentially would have become AB/DLs. My interest in diapers, and other baby things, started when I was very young, way before I had started puberty or even knew what periods were. Many people report that their AB/DL interests began in their childhood, in which case the interest would already have developed before periods were started. I don't believe that having to wear sanitary towels or tampons would alleviate somebody's need to wear diapers - if somebody had an interest in going back to child/babyhood I would say in fact it could do the opposite and increase the desire to wear diapers. Menstruating and having to wear pads/tampons is a sign that a girl is growing up and entering womanhood, whilst diapers are, to many, a symbol of childhood, so arguably having to wear pads, and thus the impending feeling of needing to grow up, might increase a girl's desire to turn to something that would make her feel little again. Wearing pads and wearing diapers feel neither physically or psychologically the same, so I would be surprised if that had any effect on female AB/DLs.