Dual SIM phones in India are really quite common; mostly due to how intra-circle roaming charges work, and the young internally-migrant population.

If your family lives in one state and you work in another, and spend time in both states, and given using the SIM from one mobile circle incurs roaming charges outside that circle (often but not always, each state is its own mobile circle, some exceptions apply); it explains why dual SIM phones are quite popular in India. Have one SIM issued in the city where your family resides, another from the city where you work. Be reachable without incurring roaming costs.

Then there's certain networks offering free/discounted/cheap "on-net" texts and/or calls which could mean if a lot of your contacts were on that network, you would have a reason to get a SIM on that network; however, given Mobile Number Portability is only just being rolled out, your only option while switching networks was to get a new mobile number; instead people would just operate with two SIMs.

Why it isn't so prevalent in the US (or Australia, or most developed world)? Most likely because the unique set of circumstances that works to necessitate (and justify) two SIMs doesn't exist.