The Miller-McCune Magazine online speculates on the scarcity of women magicians in an interesting article entitled "Why Have Women Magicians Vanished" that is subtitled "Working magicians conjure up rationales — both beneficial and baneful — for why so few women perform magic."  According to the article, their research is based upon a "survey posted on various magic Web sites and boards (and thus not meant to generalize all magicians), 220 male and seven female amateur and professional magicians answered the question: Why aren’t there more women magicians?"  I'll leave it for you to examine the sample responses that were cherry picked for the article, but the author summarizes his thoughts by saying:

Looking at the increased number of women in other traditionally all-male occupations, such as medicine or law, obscures how many still view gender differences in areas characterized by issues of power and control. Perhaps only when magic’s gender imbalance changes can we declare that discrimination based on sex has truly vanished.