Because that was the way the world we lived in worked: the work women did wasn’t important, and so it was fine for women to be better at it than men.
This reasoning is so familiar that though I can just about mange to laugh wryly when I hear such an anecdote, it almost makes me want to cry. The Russ-ian literary equivalent would be: If women are good at writing a certain kind of story, it must not be either profound or interesting and is certainly not worth reading.
Of course, that was exactly the attitude toward novels in the late 18th and early 19th century England. From the Gothic novels on, novels were a women's art form, and considered frivolous, trivial, possibly immoral and so on. Serious people -- men -- read collected sermons or poetry.
Jane Austen, who loved novels and wrote some of the greatest, has scenes when men -- guys like Mr. Collins, that idiot -- sneer at novels.
At some point, probably with Dickens, men became dominant in the writing of novels, and they became a serious art form. Though Dickens was madly popular and kind of trashy. True seriousness for the novel probably came later. At that point, women writers tended to disappear from discussions of The Novel. My friend Ruth mentioned a book she has just read, which is about ideas of The Frontier or some such thing. The author did not mention any women writers, not even Willa Cather.
(I may revise the above, after checking with Ruth. I'm not sure I'm remembering correctly.)
The same thing happens in discussions of hard science fiction. Women rarely make the lists of hard SF writers. I think there is a double prejudice operating here. One is a prejudice against the life sciences as opposed to physics and engineering.Women tend to write about biology. If you don't think biology is a real science (in this era of biotechnology and genetic manipulation) then books by Joan Slonczewski don't make the list. However, there are women who write about machinery. C. J. Cherryh and Melissa Scott come to mind at once. If they don't make lists of hard SF writers, then I think we are looking at the idea that hard SF -- real SF, serious SF -- is and has to be male.
I prefer to call hard SF "very large, hard machinery SF." I think this captures much of what is going on.