Hi there!
I think there are a lot of possible causes, but first of all, 'till the 20th century was almost impossible to a woman get an artistic career. In the last century, as well, there were many cultures where women should have a domestic life (the old-fashioned way: don't work, don't earn money, take care alone of the house-husband-children, etc). However, there are many female composers that composed music of high artistry. Clara Schumann, that you quoted, is an example. Fanny Mendelssohn, another. The great question to myself, anyway, is why their music is so little played yet. I rather believe that it is due to ignorance, but sometimes I think that must exist yet some prejudice.
In the 20th century the examples are various: Nadia Boulanger, Madeleine Tailleferre, Rebecca Clarke, Elisabeth Lutyens, Sally Beamish, Judith Weir, Madeleine Dring, Grace Williams, Grazyna Bacewicz, Sofia Gubaidulina, Galina Ustvolskaya, Pauline Oliveros, Lina Pires de Campos, Ana Torres, Teresinha de Rogatis. As usual, a very personal and incomplete list: just a few names whose music I know or like.
Of course, in the early music field you have some women that must be mentioned, for instance: Francesca Caccini, Barbara Strozzi, the trobairitz (Comtessa de Dia, in particular), and Hildegard von Bingen. Again, the list is far bigger than that.
Best wishes!