no one knows, yet.
That is a good answer instead of just guessing and believing in things with no explanation

This article looks at the possibilities and the reasons those possibilities might not be the reason.
Maybe it is a combination of all the things mentioned.

But note the first thing they consider is the use of masks and encouragement of good hygiene like hand washing.

https://www.npr.org/sections/goatsandsoda/2021/02/01/962821038/the-mystery-of-indias-plummeting-covid-19-cases



 "It's not that India is testing less or things are going underreported," says Jishnu Das, a health economist at Georgetown University. "It's been rising, rising — and now suddenly, it's vanished! I mean, hospital ICU utilization has gone down. Every indicator says the numbers are down."

Scientists say it's a mystery. They're probing why India's coronavirus numbers have declined so dramatically — and so suddenly, in September and October, months before any vaccinations began.

They're trying to figure out what Indians may be doing right and how to mimic that in other countries that are still suffering.