<p>Public transportation plays a critical role in promoting sustainable urban mobility and supporting the United Nations Sustainable Development Goals (SDGs). In medium-sized cities, conventional bus systems are often the backbone of public transport; however, in cities with populations under one million—particularly those lacking traffic restriction policies—bus services are frequently perceived as less attractive than private cars. This perception contributes to low ridership, declining service quality, and a negative feedback loop that further reduces system efficiency and usage. This study aims to identify and analyze the factors influencing the attractiveness of conventional urban bus services, with a focus on both current users and non-users. Kerman, a medium-sized city in southeastern Iran, is selected as the case study. Data were collected through structured questionnaires from 393 users and 408 non-users. Structural Equation Modeling (SEM) was employed to develop and evaluate two complementary models capturing actual usage behavior and potential modal shift. The study introduces a novel framework by operationalizing Bus Engagement as a multidimensional latent construct, incorporating frequency of use, temporal diversity, and trip purpose diversity. It further integrates key service dimensions—network, fleet, and station conditions—alongside behavioral factors such as sustainable travel attitudes. The results indicate that Bus Engagement is primarily influenced by travel behavior, sustainable attitudes, and network performance, while private car ownership emerges as a key barrier within passenger characteristics. For non-users, service improvements alone are insufficient to induce modal shift without complementary measures such as car-use restrictions. The findings provide actionable insights for designing integrated strategies to enhance bus service attractiveness and increase ridership in medium-sized cities.</p>