Why are there regional differences in research interests in mathematics and similar areas? 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.)
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.


This is the part where you are mistaken. Access to apparatus is not the only reason nor the main reason researchers like to be in proximity. The reason is that physically talking in person is often much more effective than every other remote communication method for discussing ongoing research. It is a lot easier to work on a field if you can frequently meet with other people from that field, have lunch with them, go to their conferences without it being a huge journey and get their feedback on your work directly. Also, when teaching people tend to focus on their own favorite areas of work, which leads to undergraduates becoming biased towards those areas. Undergrads themselves have a slight tendencies to remain in their own country, in which case they are likely to end up in fields popular there.

So in sum, there is one giant factor which is that in-person communication is very important. There are also many tiny factors that, together, create self-amplifying effect. Being around people with an interest makes you more likely to also develop that interest, and having an interest makes you likely to end up being near other people who share it. Local biases in interest develop as a result.