Supplier development (SD) initiatives play an important role in building buyer-supplier capacities to deal with social and economic performance. The aim of this paper is to explain the motivations behind the adoption of similar SD strategies to address sustainability (particularly social issues) occurring in local supply chains of a developing country. We compare the economic and social variants of institutional theory to investigate whether efficiency or legitimacy seeking drives the adoption (and copying) of SD practices. Twelve case studies involving private and cooperative dairies were conducted in India via semi-structured interviews. Findings revealed that both private and cooperative dairies adopt similar SD practices to build supplier capabilities and improve social and economic performance of buyers and suppliers. Private dairies imitate SD practices of the cooperatives to survive the competition; i.e., economic performance is the main reason behind their isomorphism. Cooperatives, on the other hand, take up SD practices more for legitimacy and even philanthropic reasons; though also partly note the long-term economic benefits from developing the supplier community. These findings can be used both by practitioners and researchers to gain valuable insights into the reasons driving organisational isomorphism in social sustainability practices from a developing country perspective, something rarely done so far in the literature.