It's not very uncommon that only people in some particular part of the world are working on a specific area of research. For instance, there's a considerable difference between the styles of philosophy studied in English-speaking countries and the continental Europe; the difference can be explained by that in the languages the researchers use. It is also conceivable that, in experimental sciences, doing research on some field is impossible without using special apparatuses or organisms that exist only in some specific part of the world.
However, I also observe such regional differences in research interests in mathematics (, theoretical computer science, and possibly more). In these fields the explanations I gave for the example above are not really applicable: they effectively use the same language, and you don't need anything except commodity items to do research on these fields.
I vaguely see the influences of socio-political factors, of course, but I cannot tell what exactly they are. What are they? (I would like to know how this works to make a best choice, as a graduate student, in regard to the field I work on.)