Feminism has something to do with the change, I think. But economics is also a major factor, and it has been so for centuries. In the middle ages common folk did not get married until their late twenties, in spite of the drastically shorter life spans that we have now. They did not marry until they (both men and women) had worked long enough to save money to set up a household and support themselves. Even then they did not marry primarily for love, the first issue in any marriage was whether the two people had enough resources and income between them to raise a family. If they didn't, they didn't get married and they didn't have children.
The major reason for early marriages in the United States was because pioneering a new nation could not be done singly, one needed a partner. And also a marriage meant you could grow your own helpers in the form of children.
There are many issues that go into changes like this, and changes like this have been happening for centuries.