I get what you are saying, but they decided to focus on an area, maybe a geographical one with particular relevance to the question. They might have picked a particular racial group because it is understudied as a demographic. We could generalise these findings but since they did their study in a particular group its accurate to highlight the group and not generalise. We would hope the authors follow up in other groups, even though its bleedingly obvious its generalisable, in science you should not state beyond the scope of what your experiment examines.