Metco is a volunteer program -- which explains not only why it can be race-based, but why it must be. The participating suburban communities are voluntarily diversifying their schools by accepting African-American students, but have essentially no interest in poor kids as such. An income-based program wouldn't do for them. As to other ethnic groups (being excluded from Metco), Latinos are much more controversial (and in this part of the country lack the moral imperative associated with blacks), and Asians are moving in on their own.