[DrPhil]

To build on epchick's post, a big reason why blacks and whites are used in most studies of race and ethnic relations is because blacks have long been the largest racial minority group in the U.S. To get a comparable sample size of other minorities generally requires oversampling unless the research goal is to examine Asians and Hispanics but not to compare these groups to the larger groups of "black" and "white."

People of Hispanic origin have increased in population however this includes a large population of black (race) Hispanics (ethnicity/culture), white Hispanics, and Hispanics who identify with more than one race. So for studies that are secondary analyses of census data and other datasets or for researchers who collect their own data, the 2-category system is most applicable and accurate.

Lastly, there is a rich historical interaction between people of European and African descent in the U.S. and a few other societies. Often only matched or surpassed by the history of peoples of "Native American" descent in the Americas and people of European descent.