One reason could be that we haven't detected many cases because of lack of testing. The rate of testing is very low in India, with only about 48000 tests being done as of March 31st. (According to the press release by the government, this was only 36% of our total testing capacity).
Other countries have been more proactive in testing. US, which currently has the highest number of patients, tested more than 1.2 million people as of today.