Why didnt the RBI do anything to control the depreciation of Indian Rupee?

The Central Bank of any country is entrusted with the responsibility of protecting the value of its home currency. They usually kick into action when they suspect any speculative attack on their currency by external forces (Intentional attempts to devalue a country's currency)

In this case, the devaluation of the Indian Rupee was not due to some intentional attempt by anyone. It was due to the global economic scenario and any steps they take might backfire if the global economic situation worsens.

The RBI just let the economy take its course with the exchange rate between US Dollar and Indian Rupee because there was no foul play suspected.

A point to note here is that, the RBI is closely monitoring the situation and may intervene if they feel the depreciation is too much.