Apple sales are weak in India because iPhone is overpriced in India, prices of Apple products in India are a lot higher than US. In 2015, over 100 million smartphones were sold in India.
Apple sells iPhone 6s 16 GB for $920 in India[1], while you can buy comparable Android smartphones from manufacturers like Xiaomi and OnePlus for $300 to $400
Sweet-spot price for Indian middle class is $300 to $400 and they typically upgrade in 1 to 2 years.