Background: In May 2014, the Iranian government announced it would cover all uninsured Iranians. Despite free-of-charge insurance, the studies found that the coverage still needed to be completed (93%). This study aimed to understand why certain population groups remain without insurance despite the accessibility of free-of-charge coverage. This issue is not unique to Iran,it is prevalent in many other countries where, despite free coverage, not all individuals avail themselves of it, thereby exposing themselves to risks. Methods: In a matched case-control study, 89 non-insured patients were compared with 178 hospital-based controls with health insurance (2: 1). The samples were recruited at one of the leading public hospitals in the country (Imam Khomeini Hospital Complex in Tehran) in 2019. Two insured controls were selected and matched for age (± five years) and sex for each person without insurance. A conditional logistic regression was performed to assess the magnitude of effects and the goodness of fit test used to examine the model. Results: Unemployment (Odds Ratio (OR)=8. 33, 95% Confidence interval (CI): 1. 05-50. 0), being single (OR=3. 69, CI: 1. 18-11. 55), low economic status (OR=1. 85, CI: 1. 02-3. 33) and attitude towards performance of health insurance (OR=0. 86, CI: 0. 75-0. 99) were affected not having health insurance. Conclusion: Approaches that cover the entire population may struggle to ensure no one is left without needed services. There is a need for greater focus on vulnerable groups to achieve universal health coverage conscientiously. Moreover, improved services and education can positively shape public perceptions of insurance efficacy, affecting their enrollment choices. Health sector reform