I also think it includes people from latin countries such as Italy, Spain, Portugal and France. But I don't know, I always get confused by those terms in USA.
I always thought it was people from Latin America that were descended from people of those countries, but it's hardly easy to trace that anyway. Equally, I don't understand the difference between Latino and Hispanic. I always thought those demographics were more or less the same.
In answer to the OP, I think it stems to how the slave trade operated in the US compared to other countries. Although whether this is actually the case I don't know. Coming from the UK, bizarrely I'd say most Brits (including me) know more about the US slave trade than our own. Hard to say but in the Uk the main social conflict always seems to be more rich v poor than in the US where it's more centres around race.