The most likely reason for this is the fact that sex roles have been changing in the United States and we are at a point in this process that has put more stress on women.
More than four decades ago, women typically knew what their lives would entail. They would typically be housewives and mothers. They were socialized to know that this was what they would be so there was not much reason for frustration and stress with their lives.
Since then, women’s roles have changed. Women have come to expect to be able to have careers. However, at the same time, our societal expectations have not changed with regard to work in the home. Women still end up doing most of the child care and most of the house work. They are, in a sense, expected to be everything they were 40 years ago, but they are also expected to be successful in a career. This leads to more stress than once would have been experienced. In the process women become less happy (if we can trust these surveys) than they once were.
Maybe because today most of women works for the sake of the finacial support in eddition to being a mother and a wife, which takes so much effort and less time for themselvs. Another point to add is that the more life get complecated the more they become desperate with their lives.
Although I think that women in the past were not that much of a happy women, cuz they ment nothing ( of course that is according to the ancient history).