Conventional wisdom is that social capital in India is low because of our historical caste system. By placing people in a rigid hierarchy, and giving some people privileges over others just because of the families they were born into, the caste system prevented people from cooperating as well as they would in a more equitable society – that is what conventional wisdom says.
However, a point that we cannot miss is that despite the caste system placing a hierarchy on people, people from different castes did regularly cooperate and trade with each other. In fact, with caste being tied to hereditary professions, people had little choice but to regularly interact and trade with people from other castes. And this inevitably created social capital.
Putting it differently, the result of the caste system was an unequal but stable society, and this stability led to reasonably good social capital (history might be biased given it was written by people from certain castes, but we don’t see many instances of caste riots or clashes from over 200 years ago). You can think of it as a stable society with “handicaps”, where some people were privileged over others (in fact, there was a hierarchy of privilege), to the extent that it was okay for some people to abuse others in various ways.
Over the last 150 years or so, the caste system has been (rightly) challenged, and we are seeing various movements towards a more equal society. One side effect of this has been that the (unequal) equilibrium that had existed has been disturbed, leading to caste-based antagonism and a fall in social capital.
We are in the process of moving from one (unequal) equilibrium to another (more equal) equilibrium, but until we get there, existing beliefs and biases will continue to be challenged, which means some sets of people will continue to be suspicious of others, and there will be mistrust and thus low social capital.