When I was being officially educated (as against the real process, which is still going on) I went to a Science and technology university college, and maybe 2% of the student body was female. As far as I can remember, not one member of the teaching staff was.
Now, this was the sixties, so we were out of the "oh, don't worry your pretty little head about education; you'll get married and pregnant and all that time will have been wasted." mentality (just – some is still hanging around to this day) but there was still a "don't get your pretty little hands dirty in all that nasty grease" holdover. Most of the few females were in biological sciences, or mathematics; my university girlfriend was the sole representative of her gender in three years of civil engineers.
And the streaming had started seven or eight years earlier, specialisation from the eleven plus, "Oh, stick to letters, to humanities; they won't be hiring any women for that type of job.“
There's a lot of inertia in a system like that. Not deliberate sexism, but rôle models, ageing teachers who knew that women had taken on a lot of masculine tasks during the war, but now they were no longer needed, surely they could go back to their true destiny of house and children, and perhaps read an occasional book, maybe even write one – and teach primary school if they are incabable of finding an adequate mate to provide.
And I was already reading the colours of space, memoirs of a Spacewoman, the ship who sang…
What is the average age of an author? It's possible that among the younger generation this gentle discrimination is by now wearing out, and it's only the older writers who are biasing the statistics. I know that the incredible gender imbalance in scientific and engineering colleges is much reduced, if not eliminated by now (and it is no longer mandatory to be illiterate to study in one, either).
For Science Fiction requires at least a minor grounding in a science, and in scientific method. And from this our education system, backed by social and parental pressure, was trying to shelter its delicate beauteous flowers.