Because there was no point in having a middle-level of armor. The ship-caliber guns were so large that anything less than the super-heavy armor they put on vital parts would be almost as useless as no armor. Also, fully armoring the ship to that heavy level would have added hundreds of thousands of pounds to the ship, slowing it down and making it far less maneuverable. They realized this and decided to compromise, heavily armoring vital parts and essentially leaving the non-vital parts unarmored. So they called it "all or nothing" because the parts of the ship could either have "all" the armor and be protected from fire or have "nothing" and sacrifice armor for weight reduction and maneuverability. Hope this helps!