Strong family structures exist all over the world, but the nuclear family developed only in western Europe during the last millennium. A couple of the big causes were the medieval economic system of manorialism, and the Christian abolition of cousin marriages, polygamy, and divorce. These forces combined to break down tribes and clans, and to organize farms around self-sufficient nuclear families. The serfs developed a future time orientation, a belief in rule of law, and a regard for family autonomy.
Even today, Europe is divided by a Hajnal line, with late marriages on the west and early marriages on the east. Early marriages are more likely to be arranged by the parents, and late marriages are more likely to have a greater investments in a smaller number of children. There is more explanation on the hbd* chick blog.
Western Europe is also the origin of many great ideas, such as individualism, private property, personal freedom, and capitalism. America got these concepts from western European settlers, and the concepts are most appreciated in cultures with nuclear families.
America is now in the midst of grand social experiment to abolish the nuclear family. It is being done with changes to marriage law, government incentives, public moral attitudes, and immigration policy. The trends started in the 1960s, and have now reached the point where the President has been reelected in large part based on promises to accelerate those trends.
In last month's election, most of the nuclear families voted for Mitt Romney. Most of the others voted for Pres. Obama.