India is the home of the old meme "he's over qualified for blue collar so has to drive a taxi" because indian general education is world class and a massive amount of Indians go into further study. Of course that doesn't explain exactly why india people are good at chess but explains a little.
Indian education is awful
indian education is strict but extremely effective
yes extremely effective at making people stupid
All these reasons aside, Indians are good at chess because chess is popular in India. Smart or not, someone raised by indian parents to be a chess player will be a good chess player...