Regan Penaluna (St. John's University) offers the following explanation (this is subscription access only to read the whole thing):
Ask a student to name philosophers from history, and she will probably rattle off Plato, Aristotle, Descartes, Locke, and Kant. The cliché that philosophy is the study of dead white men persists for a reason. If women have trouble finding a female role model in their departments, then they will have even more trouble searching for one in the canon.
The predicament is more acute when we turn to what great philosophers have to say about women. Take, for instance, Aristotle's claim that women exist to do household work so that men can participate in political life. Locke, who was considered radical in his day for arguing that parents ought to share authority, nonetheless undercuts this when he says that a wife should subordinate her judgment to her husband, who is "abler and stronger." Rousseau insists on natural equality, yet claims that women are to be dependent upon men. He also tells us that women are simply not that interesting: "Women in general do not love any art, are not knowledgeable in any, and have no genius." Nietzsche praises Napoleon's statement that women ought to stay out of politics. As if that were not enough, he also complains that women are bad cooks who have "delayed human development."
Putting to one side the sophomoric cariacture of Nietzsche (on this issue, see the important paper by Maudemarie Clark), I wonder how plausible readers find this explanation. That women are among the most important contemporary scholars writing on Plato, Aristotle, Descartes, Locke, and Kant (and Nietzsche: vide Clark)--indeed, sometimes the most important scholars--obviously isn't incomptabile with the hypothesis in question, namely, that an overwhelming male historical canon, combined with sexist remarks by many of these philosophers, put off female students from the field. But is the hypothesis a plausible explanation? Signed comments (full name in the signature line) will be strongly preferred, but students who use a valid e-mail address may post without a full name in the signature line.