As for the women, they are putting career first, delaying any thought of marriage, then baby hunger kicks in during the 30s as they see their window of opportunity closing, but still remain picky about their choice of men, then finding there is a shortage of males who want to marry older, “independent minded” (translate: kinda bitchy) career women.
These are descriptions of proximal causes, but what are the underlying dynamics leading to them?
I attribute a large portion of this change to TV and movie portrayals of family life. In the 50's and 60's you had Leave it to Beaver and Father Knows Best. Men were the leaders of the household, and the wives were strong women who supported their men. A unified front.
Starting in the 70's, men were increasingly portrayed as buffoons. Only the mothers had the right answers, the fathers were to be laughed at. We now have several generations of women who believe they're to be the strong leader of the family, and the husband is to be an inconsequential amusement. The disconnect comes as these Hollywood farces don't come close to representing real families and real life.