Reasons:
1) Smartphone market is still nascent in India. Better to enter sooner rather than late like in other western countries.
2) Realistically, you have a bigger market to sell to.
3) Nokia is very popular in India. So much that, it had been consistently rated by people as the top and most trusted brand amongst all products in India. For 10 years now.
4) India didn't get all the first generation WP7 devices.
5) iPhone is not affordable for many. It's way way priced higher. iPhones are launched in India 10 months after US launch and sold twice the price (unlocked).
6) Careers decide nothing. It's a direct market where phones are NOT sold through careers. Most careers don't even sell a single phone. Not even one. They are mobile service providers and not mobile phone providers. So WP7 has a fighting chance to appeal to people directly.
7) Mobile service contracts are unheard of. If I decide and can afford to buy a new phone today, I buy today not till a stupid contract is over like here in US. (Edit: I personally treat mobile contracts and exclusivity as stupid, but your opinion may differ. I'm aware that contracts discounts the phone and I use a ATT Focus.)
Still almost all smartphones including Android, WP7 etc. except for few are launched late in India. It's mostly with the price point. After 4-6 months of selling in western countries, you have a margin to price it less in Asian countries like India.