"Is Marriage for White People?" attempts to answer that question. As I remember it, the main answer was that black women don't want to date outside their race.
http://www.amazon.com/Is-Marriage-White-People-American/dp/B...
http://www.amazon.com/Is-Marriage-White-People-American/dp/B...
What about social pressure (you are a "race traitor" fords ting non blacks). Is it an anti white thing? Is the proportion of Hispanic dating higher? What about comparing across cultures - is the incidence of dating cross race lower than in say the UK?
The fact that we are raising race to its own singular cause for this sort of marriage-limiter shows how much race issues have a grip on our societies, but it's interesting to think around the subject.
Still would be interested to know how one can find a epidemiological "natural" experiment to solve this.
It's also pretty ordinary for people to prefer mates that have a broadly similar life experience and perspective as themselves. It's easier to sustain a relationship when you have more common ground.