I would always simply point out that St. John was the most beloved and devoted of all the disciples of Jesus Christ, but he wasn’t the one who Christ selected to be “the Rock” on which He would build His church. It was PETER — a badly flawed, ill-tempered, and cowardly man — who was chosen for this role.
Christ knew better than anyone else that the world needs tough shepherds to fend off the wolves that seek to scatter and devour His sheep.