Wait. I have the definitive answer: because the US isn't Europe and doesn't have a social welfare system.
I can't see how the study actually proves anything. It does help to eliminate some factors, but whether or not these had to be eliminated from the discussion would depend largely on whether or not you accepted the theoretical assumptions that went into setting up the analysis.
It seems to me that the political institution side of the argument is far stronger than the race one. Even if the category 'poor' is disproportionately 'black', 'whites' as a 'race' make up the largest group of poor by far. Also, we are now into theorizing about things that largely don't exist (the US's social welfare system, socialist parties, and even voter responsibility for public policy, like, when did it ever come up to a vote anyway? etc.). Always pretty iffy.
If it were up to voters (which brings up voter registration and party participation as determinants) or the perceptions of elite policy makers, could we sell social welfare on the basis that it would benefit far more whites than other racial categories?
I'm more set on political institutions and government as the explanation (has there ever been a more f-ed up structure than the US's federal government?).
The US has a right-wing two-party system that represents an extremely narrow range of political views, skewed very far right, very laissez faire in terms of the relationship of the government to the individual and non-economic society. It makes it practically impossible for a group on the left, even center left, to sell their ideas at the national level. Such groups do not go into a parliament and make compromise deals and coalitions in order to exercise power. The huge mess the US has made of national health insurance is the best possible example.
As for the political immaturity of the country, a little 20th century history helps us understand that. At least two major periods of red scares, violence against unions, Sacco-Vanzetti.
What's scary is that US globalism is all too often about re-making the world to fit the US pattern. When a Dutch person says to me, We need more flexible labor markets. When a German laments that his company can't compete globally because of high wages and benefits. It's not difficult to see where this is coming from. If Europeans think Europe needs to be more like America, god help them. I get this all the time in Japan too. The Japanese are now on a 'we need to be more like the US' kick. I always start such discussions with, fine, your health insurance ends today. No more 5 dollar fees at the health clinic (which you visit everytime your nose drips or you have a cough). No more annual health exams paid for by your insurance. Now how do you feel? Most can't believe I'm telling the truth.
Charles Jannuzi