Both Hindi and Urdu are victims of policies not connected to them. Urdu became the language of Islamist supremacism/separatism, and Hindu writers deserted it [many Hindus wrote in Urdu in the pre-1900 days]. This essentially sealed the fate of Urdu in India, and it lost traction.
Hindi became the language of linguistic supremacism and economic discrimination. The false idea was planted that Hindi was needed for Hindu unity and the government tried to destroy languages in south/east India. This turned away south and east Indians from the Hindi language.
In fact, if the government had not been so desperate to introduce Hindi, many south and east Indians would have learnt Hindi as they do other languages, in which they have to communicate. In the pre-1940 days, Hindi was learnt quite avidly in [the then] Madras Presidency.
But the supremacist arrogance and economic discrimination against those who didn't know Hindi ensured that backs were put up. Normally tolerant people became stubborn opponents of Hindi. I suspect that Hindi will suffer the fate of Urdu in south and east India.
