BACKGROUND: The National Health Insurance Scheme (NHIS) in Nigeria was launched in 2005 as part of efforts by the federal government to achieve universal coverage using financial risk protection mechanisms. However, only 4% of the population, and mainly federal government employees, are currently covered by health insurance and this is primarily through the Formal Sector Social Health Insurance Programme (FSSHIP) of the NHIS. This study aimed to understand why different state (sub-national) governments decided whether or not to adopt the FSSHIP for their employees. METHODS: This study used a comparative case study approach. Data were collected through document reviews and 48 in-depth interviews with policy makers, programme managers, health providers, and civil servant leaders. RESULTS: Although the programme's benefits seemed acceptable to state policy makers and the intended beneficiaries (employees), the feasibility of employer contributions, concerns about transparency in the NHIS and the role of states in the FSSHIP, the roles of policy champions such as state governors and resistance by employees to making contributions, all influenced the decision of state governments on adoption. Overall, the power of state governments over state-level health reforms, attributed to the prevailing system of government that allows states to deliberate on certain national-level policies, enhanced by the NHIS legislation that made adoption voluntary, enabled states to adopt or not to adopt the program. CONCLUSIONS: The study demonstrates and supports observations that even when the content of a programme is generally acceptable, context, actor roles, and the wider implications of programme design on actor interests can explain decision on policy adoption. Policy implementers involved in scaling-up the NHIS programme need to consider the prevailing contextual factors, and effectively engage policy champions to overcome known challenges in order to encourage adoption by sub-national governments. Policy makers and implementers in countries scaling-up health insurance coverage should, early enough, develop strategies to overcome political challenges inherent in the path to scaling-up, to avoid delay or stunting of the process. They should also consider the potential pitfalls of reforms that first focus on civil servants, especially when the use of public funds potentially compromises coverage for other citizens.