Why can the US government successfully run a massive grocery chain for the military (commissaries), but municipal-run grocery stores for the public often fail?

I recently watched a video about the Defense Commissary Agency (DECA) and how it provides groceries to military families at ~23% savings. However, when cities try to open "government-owned" grocery stores to fix food deserts, they often struggle with thin margins and eventually close.
From an economic perspective, why does the federal government-run model work while local ones don't? Is it just a matter of scale, or is there a fundamental difference in how they are subsidized and managed?