Why do so many women choose to write short stories instead of novels?
It would be interesting to know what gives you the impression that women writers are more drawn to the short story genre than to the novel.
Most women writers have, in fact, been successful at writing both. Examples of this are Harriet Beecher Stowe, Virginia Woolf, Alice Munro, Muriel Spark, Isabel Allende, Sara Gallardo, Margery Allingham, Emilia Pardo Bazán, Doris Lessing, Willa Cather, Martha Lynch and Margaret Atwood among many others.
There are also women novelists who have not written short stories, such as Siri Hustvedt, Iris Murdoch, Susan Sontag, Simone de Beauvoir, Grazia Deledda, and others. If anything, there have been more women writers that have authored novels and a small number of short stories than those who have devoted to writing short stories exclusively.
The choice of genre is independent from gender. The purpose of a short story is to convey a fragment of real or imaginary life through a relatively simple plot, little or no character development, and the greatest economy of resources. It is up to the writer to decide whether his/her work will be best exploited as a short story.
On the other hand, a novel is a much longer work of fiction, with a complex plot and subplots, a number of round and flat characters, and a clear organization that varies depending on the techniques the author prefers (linearity, flash forward, flashback, stream of consciousness, and others, all of which can be combined in one single work.) Novels may also involve more than one point of view -i.e. some parts or chapters may be written in the first person, for example, and others may be told by an omniscient narrator, though further POVs are possible.) When the subject matter demands such variety of resources, the author will prefer to present it as a novel rather than as a short story.
Thus -allow me to insist on this- rather than the author's gender, what determines the choice of genre is the content that the author wishes to develop.