The thesis of this paper discussed on Life Matters is that women are waiting longer because there is now a greater potential payoff from doing so. When all the potential males earned pretty much the same income there was little to be gained (financially) in Waiting for Mr. Right.
Increasing inequality, say the authors, explains about 30% of the marriage rate decline in the US over the last few decades. They purport to show that this is not due to the labor force decisions of women in response to rising income inequality, or to the marital decisions of men in response to rising income inequality.