Indigo cultivation in India declined by the beginning of the 20th century because of

Indigo cultivation in India experienced a significant decline by the beginning of the 20th century primarily due to economic and technological factors. The most crucial reason was the invention and mass production of synthetic indigo dyes in Germany during the late 19th century. This technological innovation made natural indigo cultivation in India largely unprofitable in the global market, as the synthetic dye was cheaper and easier to produce. While peasant resistance to the oppressive conduct of planters (like the Indigo Revolt of 1859-60, which falls outside the timeframe given in the question for the decline by the beginning of the 20th century) certainly occurred and highlighted the issues, the final and decisive blow to the industry's profitability and subsequent decline was the emergence of artificial dyes. Neither national leaders' opposition nor government control were the primary drivers of this specific decline by the 20th century.