The persistence of intrastate conflicts in West and Southern Africa continues to challenge ECOWAS and SADC’s relevance as agents of regional security management. While these regional organisations (ROs) have instituted security management mechanisms, they do not always utilise these in response to intrastate patterns of regional security threats. In this regard, this study seeks to answer the pertinent question: Why are ECOWAS and SADC not always capable of responding to regional security threats? By asking the why question, this study seeks to understand under what conditions these ROs are able or not to respond to security threats in their respective regions. Most studies on regional security in Africa tend to examine these ROs as self-sufficient actors, focusing more on their internal institutional challenges to arrive at generalised conclusions that categorise these organisations as weak. While this provides relevant insights into endogenous challenges, the literature does not give enough attention to how a complex mix of endogenous and exogenous factors contribute to undermine or bolster their capacity in given specific cases. To fill this gap, this study employs an inductive approach to understand ECOWAS and SADC’s capacity in regional security management, by conducting two qualitative case studies that question why ECOWAS was unable to respond to the 2012 Tuareg insurgency in Mali and why SADC was able to respond to the Cabo Delgado insurgency in Mozambique. The resulting theoretical framework suggests that to understand the capacity of these ROs, it is imperative to account for the interactions between four factors: institutional design, state consent, extra-regional influence, and regional power influence. The framework anchors on theoretical insights from subaltern realism and the African Security Regime Complex (ASRC) literature to explain the interactions, illustrating how and why multiple actors’ diverging orientations, preferences, and interests result in frictional relationships that undermine ECOWAS and SADC. Relying on data from semi-structured interviews and analysed documents, this study argues that ECOWAS and SADC’s response capacity is often undermined by intra and interorganisational frictions attributable to feelings of suspicion and mutual distrust between regional and extra-regional actors built upon colonial legacies. In the Tuareg case, Mali, Algeria, France, and the US resisted an ECOWAS-led response but preferred alternative security arrangements that they perceived to best serve their respective national interest. In the Cabo Delgado case, SADC was able to respond despite Mozambique’s disagreement with SADC on the regional character of the conflict and resistance towards a regional intervention, as it preferred to seek solutions through private militaries and bilateral security arrangements. The study also argues that the influence of regional powers– Nigeria in ECOWAS and South Africa in SADC– is instrumental in supporting ROs to circumvent these challenges and build regional convergence towards regional responses. However, their influence is often constrained. While Nigeria’s influence was constrained and unable to successfully support ECOWAS in circumventing the frictional challenges in the Tuareg case, South Africa was able to overcome the constraints to its power influence, thus allowing it to successfully support SADC in overcoming the frictional challenges. This explains why SADC was able to respond to the Cabo Delgado insurgency and why ECOWAS was unable to respond to the Tuareg Insurgency.