Sustainable stormwater management (SSWM) is widely promoted for urban sustainability, yet decisions often weigh modeled performance and measurable evidence more readily than the governance and relational conditions that shape delivery. Here, we examine how practitioners judged the prospective decisiveness of such conditions for future SSWM in Malmö and Östersund, Sweden. Using an exploratory sequential mixed-methods design, we combined retrospective interviews with a Best–Worst Method prioritization workshop. Interviews identified 40 factors that have shaped stormwater management since the 1960s, which were synthesized into 9 key factors for prioritization. Participants ranked External Collaboration as most decisive and Technological Innovation and Adaptation as least decisive. The results suggest that practitioners judged that future SSWM decision-making depends on expected system performance as well as delivery conditions, including coordination, policy authority, organizational capacity, land-use negotiation, funding continuity, and long-term maintenance. Our study offers a context-grounded approach for making delivery conditions explicit in future SSWM decision support.