Sexism, pure and simple.
Some men felt there were "men's jobs" and "women's jobs" and that women should not be entering the "male" sphere.
They felt that working would somehow make women less lady-like and less refined.
This article should help:
http://www.warandgender.com/wgwomwwi.htm
Winston Churchill's mother wrote a book about women during WW1. Here it is:
http://libraryautomation.com/nymas/jenniechurchill.html