It's about the level of education, especially around family planning/birth control, how easily women can refuse, what other avenues there are for people to keep themselves satisfied besides sex, and what people can afford.

The well off, well educated city dwelling portion of the Indian population is approaching similar standards to the West, couples have maybe 1 or 2 children, women have more freedom to refuse, while the cost of raising a child to have a better life than they had is still low enough.

The poorer segments are gradually getting there, but it takes time.

I hope that the case isn't that you're not actually curious and are trying to imply that the population needs to be made to decline by force.