In history lessons I always had two types of teachers: ones who made it seem that slavery only dealt with Africans, and they were quite apologetic for it. Then I had the others who made a point to say that "blacks weren't the only slaves, plenty of white European people were slaves too". I found both methods of approaching the subject problematic for different reasons, but being AA myself, I found myself in a tough spot. I'm not one for promoting skewed data or focusing on one aspect of an issue, but if I were to bring up the fact that other races were enslaved to my peers, I'd be accused of "downplaying" the struggles our ancestors lived through during slavery.
I think the NAST is focused on so heavily because, like Simon said, the large amount of descendants of African-American slaves. Also because the huge affect it had on our country socially, politically and economically for hundreds of years afterwards, some of which can be seen today.