Question

why were Indian farmers hesitant to grow Indigo

Answer 1

Because they had to more and more careful because late will happen then their soil and indigo will be damaged.
Farmer don't want grow indigo because it can lose the fertility of soil.

Answer 2

Simple because once when indigo is grown in a land the soil loses it fertility.

And in the late time when India was independent the farmers where forced to grow indigo but don't get the appropriate wages as well as they even can't grow the crops of their choice.