Species abundance distributions (SADs) are a fundamental pattern in ecology, describing the distribution of species abundances within ecological communities. However, their underlying causes remain debated. Classical explanations invoke either stochastic processes, as in neutral theory, or ecological differences among species, as in niche-based approaches. To investigate the processes that can generate SADs, we developed a mechanistic framework that integrates habitat suitability, negative density dependence (e.g., through frequency-dependent predation), and demographic stochasticity within a single analytical model. In this framework, species differ in their habitat suitability, while density-dependent regulation limits the dominance of highly abundant species and demographic stochasticity introduces random fluctuations in population dynamics. This combination yields an explicit analytical expression for the SAD and establishes a direct link between community-level abundance patterns and species-specific habitat suitability. The framework provides a unified interpretation of classical SAD forms. When interspecific variation in habitat suitability dominates, the resulting distributions tend to be lognormal-like, whereas high demographic stochasticity produces patterns that resemble those predicted by neutral models. By bridging niche-based and neutral perspectives, our model can help to identify the ecological mechanisms shaping SADs by generating testable predictions that relate distributional shape to measurable ecological parameters.