Because they have more important things to do

Rico says just ask them, they'll tell you. But Noam Cohen has the story in The New York Times:

In ten short years, Wikipedia has accomplished some remarkable goals. More than 3.5 million articles in English? Done. More than 250 languages? Sure. But another number has proved to be an intractable obstacle for the online encyclopedia: surveys suggest that less than fifteen percent of its hundreds of thousands of contributors are women.
About a year ago, the Wikimedia Foundation, the organization that runs Wikipedia, collaborated on a study of Wikipedia’s contributor base and discovered that it was barely thirteen percent women; the average age of a contributor was in their mid-20s, according to the study by a joint center of the United Nations University and Maastricht University.
Sue Gardner, the executive director of the foundation, has set a goal to raise the share of female contributors to 25 percent by 2015, but she is running up against the traditions of the computer world, and an obsessive fact-loving realm that is dominated by men and, some say, uncomfortable for women.
Her effort is not diversity for diversity’s sake, she says. “This is about wanting to ensure that the encyclopedia is as good as it could be,” Ms. Gardner said in an interview. “The difference between Wikipedia and other editorially created products is that Wikipedians are not professionals, they are only asked to bring what they know. Everyone brings their crumb of information to the table,” she said. “If they are not at the table, we don’t benefit from their crumb.”
With so many subjects represented— most everything has an article on Wikipedia— the gender disparity often shows up in terms of emphasis. A topic generally restricted to teenage girls, like friendship bracelets, can seem short at four paragraphs when compared with lengthy articles on something boys might favor, like toy soldiers or baseball cards, whose voluminous entry includes a detailed chronological history of the subject.
Even the most famous fashion designers like Manolo Blahnik or Jimmy Choo get but a handful of paragraphs. Consider the disparity between two popular series on HBO: The entry on Sex and the City includes only a brief summary of every episode, sometimes two or three sentences; the one on The Sopranos includes lengthy, detailed articles on each episode.
Is a category with five Mexican feminist writers impressive, or embarrassing when compared with the 45 articles on characters in The Simpsons?
The notion that a collaborative, written project open to all is so skewed to men may be surprising. After all, there is no male-dominated executive team favoring men over women, as there can be in the corporate world; Wikipedia is not a software project, but more a writing experiment, an “exquisite corpus”, or game where each player adds to a larger work.
But, because of its early contributors, Wikipedia shares many characteristics with the hard-driving hacker crowd, says Joseph Reagle, a fellow at the Berkman Center for Internet and Society at Harvard. This includes an ideology that resists any efforts to impose rules or even goals like diversity, as well as a culture that may discourage women. “It is ironic,” he said, “because I like these things— freedom, openness, egalitarian ideas— but I think to some extent they are compounding and hiding problems you might find in the real world.” Adopting openness means being “open to very difficult, high-conflict people, even misogynists,” he said, “so you have to have a huge argument about whether there is the problem.” Mr. Reagle is also the author of Good Faith Collaboration: The Culture of Wikipedia.
Ms. Gardner, citing an example that resonates with her personally, pointed to the Wikipedia entry for one of her favorite authors, Pat Barker, which was a mere three paragraphs when she came across it. Ms. Barker is an acclaimed writer of psychologically nuanced novels, many set during World War One. She is 67 and lives in England.
By contrast, Niko Bellic had an article about five times as long as Ms. Barker’s at the time. It’s a question of demographics: Mr. Bellic is a character in the video game Grand Theft Auto IV; he is 30 and a former soldier.
The public is increasingly going to Wikipedia as a research source: according to a recent Pew survey, the percentage of all American adults who use the site to look for information increased to 42 percent in May of 2010, up from 25 percent in February of 2007. This translates to 53 percent of adults who regularly use the Internet.
Jane Margolis, co-author of a book on sexism in computer science, Unlocking the Clubhouse, argues that Wikipedia is experiencing the same problems of the offline world, where women are less willing to assert their opinions in public. “In almost every space, who are the authorities, the politicians, writers for op-ed pages?” said Ms. Margolis, a senior researcher at the Institute for Democracy, Education and Access at the University of California, Los Angeles.
According to the OpEd Project, an organization based in New York that monitors the gender breakdown of contributors to “public thought-leadership forums”, a participation rate of roughly 85-to-15 percent, men to women, is common, whether members of Congress, or writers on The New York Times and Washington Post Op-Ed pages.
It would seem to be an irony that Wikipedia, where the amateur contributor is celebrated, is experiencing the same problem as forums that require expertise. But Catherine Orenstein, the founder and director of the OpEd Project, said many women lacked the confidence to put forth their views. “When you are a minority voice, you begin to doubt your own competencies,” she said, and that her group had persuaded women to express themselves by urging them to shift the focus “away from oneself— ‘do I know enough, am I bragging?’— and turn the focus outward, thinking about the value of your knowledge.”
Ms. Margolis said she was an advocate of recruiting women as a group to fields or forums where they are under-represented. That way, a solitary woman does not face the burden alone.
Ms. Gardner said that for now she was trying to use subtle persuasion and outreach through her foundation to welcome all newcomers to Wikipedia, rather than advocate for women-specific remedies like recruitment or quotas. “Gender is a huge hot-button issue for lots of people who feel strongly about it,” she said. “I am not interested in triggering those strong feelings.”
Kat Walsh, a policy analyst and longtime Wikipedia contributor who was elected to the Wikimedia board, agreed that indirect initiatives would cause less unease in the Wikipedia community than more overt efforts.
But she acknowledged the hurdles: “The big problem is that the current Wikipedia community is what came about by letting things develop naturally — trying to influence it in another direction is no longer the easiest path, and requires conscious effort to change.” Sometimes, conscious effort works. After seeing the short entry on Ms. Barker, Ms. Gardner added a substantial amount of background. During the same time, Niko Bellic’s page has grown by only a few sentences.