because india had many people and a lot of wealth. To the british empire they were considered "jewel in the crown". Due to the spices, cotton, opium. That the British would sell to China for tea.
India had all sorts of goods that the British wanted. These included things like spices, textiles, cotton, and the opium that the British would sell in China to be able to buy tea. Because India had so many people and so much wealth, it was the "jewel in the crown" of the British Empire