Why, in Woolf's view, did Elizabethan women not write poetry?
In general, Woolf is saying that women the limited amount of women's writing is due to the patriarchal history; but more to the point, the lack of women writers is more directly due to the lack of materials. Woolf says that women need a room of their own, education, income and so on: they need the same "materials" that men have always had access to. A Room is just one thing, one material thing. There would have been more women writers, historically, had there been a more gender-equal society; and with a more equal society, women would have had just as much access as men to the materials, tools and resources that are conducive to writing as well as other arts and trades.
In one of the more famous sections, Woolf proposes a fictional character, Judith, who is Shakespeare's sister. Woolf claims that, had there been a woman with Shakespeare's abilities, she would not have been allowed the same opportunities because she was a woman. So, it's not that Elizabethan women didn't write or didn't care to write; it was the fact that the predominance of the idea of men's superiority over women infested all aspects of life, making it much more difficult for women to have access to the tools and information/education of the trade.