I would guess typical passenger planes have significantly smaller cargo stores than cargo planes so the layout may not be good for efficient cargo transport (ou

I would guess typical passenger planes have significantly smaller cargo stores than cargo planes so the layout may not be good for efficient cargo transport (outside of course... people as cargo). You'd allocate the volume in the aircraft very differently I imagine for cargo than passengers. I also suspect passenger planes were already using any leftover cargo from passenger cargo storage for commercial cargo transport to begin with. Two potentially very different optimization problems.