At the current rate of iPhone sales in India (< 0.2% of the population a year), it would take more than fifty years to hit 10% of today's population figure.
It's all about price and subsidies. Doesn't matter if it's a poor or rich country, people don't like paying a lot upfront. Most people are going to buy the best device they can for under ~$250 out of pocket.
India doesn't have subsidies, so the normal $570 iPhone price is too much for most people (this would be true even in the USA!). Trade-in programs and loans are how Apple has managed to boost their sales to even the relatively small amount it has now in India.
Ironically, it's the richest countries that have the most subsidized prices which lower the perceived device cost. I.e it's where the iPhone is priced under $200 (USA, etc) or even free (Japan), that it rises above ~15% marketshare.