There's so much emphasis (from comments) on information technology services contributing to this. This is a recent phenomenon. Growth due to IT services was extremely concentrated in a few cities and most took their family there.
India was never a nation state before becoming a federal republic. Poverty was not widespread uniformly over the subcontinent. Over-reliance on agriculture and lack of trade meant factors like climate patterns disproportionately affected people in the lower social order.
Government definitely played a role in establishing energy infrastructure, educational institutions, introducing healthcare schemes and before that, smart people played an even bigger role with the constitution.
India's growth can be traced to a small list of landmark decisions which caused chain reactions.
- 1950 Constitution and enforcement by institutions
- 1954 Midday Meal Scheme
- 1956 Non-Aligned Movement
- 1961 Green Revolution
- 1970 Operation Flood
- 1991 Economic liberalisation
Looking back, just liberty guaranteed by Constitution wouldn't have worked because of the caste system.
I can add more if someone's interested.
- itspi