High-quality early childcare structures are effective policy levers for reducing inequalities. However, underprivileged populations in Western countries access them the least. The reasons underlying this unequal access have been studied, but not systematically. In addition, many demand-side factors (e.g., information level of families, decision heuristics) have been understudied, and have not previously been theorized in this field of study as contributing to this unequal access. Only the role of preferences has sometimes been considered as contributing to this unequal access. This article fills these gaps by making two major contributions to the literature. At the theoretical level, it proposes a comprehensive classification of the different factors that can underlie these dynamics. At the empirical level, it reexamines the existing evidence, evaluating the proposed factors' relevance through a PRISMA mixed-methods systematic review of the empirical literature. Relying on a cross-validated impact score and p-curve analyses, we find evidence that demand-side factors could contribute as much as supply-side factors (e.g., shortage of places and fees) to this unequal access. However, current public policies often only target supply-side factors. If future research confirms our results, new public policies should be created considering these demand-side factors. Otherwise, structural reforms' effectiveness could be severely limited.