Zeppelins have all the slow speed of ships and all the inefficiency of flight. Remember, planes spend very little energy on keeping themselves in the air (hence why unpowered gliders can stay aloft for hours), their engines are for overcoming drag, pushing large volumes of air out of the way. Airships move at much lower speeds, but they have immensely larger cross sectional areas. They don't need to push it as fast, but to travel the same distance they must push substantially more air out of the way. Airships are great if you need long endurance, for example for aerial photography, but they're a terrible option for transportation.