1) Feminism brought them equality in the work place. Thus they don't need to depend on men for economic survival. In countries where this has not happened, she lives with her parents until she finds a husband. This is turn means Western women can be extraordinarily picky about choosing a man since "settling" is not an economic necessity as it was in the past. For many if they don't find exactly the guy they want, they would rather focus on their careers and stay single indefinitely("never settle!"). The feminists chant "hoorah! liberation!" but the end result will be plunging marriage and fertility rates like you have in Scandinavia. Obviously a nation with a negative birth rate is not sustainable in the long run so nature will get the last laugh, as will those "primitive" societies were women actually reproduce.
2) The stigma of being a spinster in Western society has vanished. It's simply no big deal for her to be 35+ and unmarried. Sure many of them may long to meet a man of their dreams, get swept off their feet and settle down, but nobody is going to give them a hard time about it if they don't. This is especially true in the major cities. She will filll that empty hole in her life with career ambition, yoga and pets rather than lower her standards and accept a man which is not her ideal.
3) The importance of career over family. This ties into number 1. The modern woman places the importance of career and "independence" as sacrosanct. Becoming a mother and a wife is secondary. It's something she'll think about after she's landed that 6 figure job at a legal firm. Housewives and women that put family first are unambitious anachronisms. A man is not only competing against other men for her attention, he's competing against her career ambitions.
http://www.theatlantic.com/magazine/arch...ll/309020/
4) Demographics. Men have traditionally had a higher death rate(due to wars, riskier behavior, harder work, etc) than women. Yet nature compensated by giving males a higher incidence of birth. Technology and modern medicine has thrown off this balance. Fewer men are dying at a young age. We don't have tens of millions of dying in wars anymore. Syphilis doesn't kill a man anymore, most men work in offices rather than sawmills and mines. So within the 18-30 demographic when women are at their prime years of procreating, you have an oversupply of men in Western countries. And with more men being unmarried into their 30s and 40, they are also competing with 20 something men for desirable 20-something women.
5) Women don't want men as bad as men want women. I don't care what anyone says, women are not as horny as men. They may enjoy sex as much as men, but they aren't thinking about it 24hrs a day like we are. They aren't constantly looking for innovative new ways to find it. Women can go for long stretches of time without sex that would break the soul of a man. A desirable woman in her 20s doesn't have to dedicate even a second of her day thinking about how can she meet men.
At the end of the day, I'm not sure Western women have as much choice in men as we think. All these things have given them the *illusion* of greater choice which makes them willing to hold out well past their prime reproductive years. It would seem to me that in some ways their choice has actually diminished since they have very few men to choose from that they consider desirable by their inflated standards. They have the choice to hold out longer due to financial independence but most of them will not find the alpha male of their dreams. An increasing number of them will never have kids and never marry.