It's a very valid question. It gets asked on a regular basis, and it should be. It's historical in many ways. It has to do with investment in the technology.
Before the Second World War airships and airplanes were neck-and-neck as to which was going to be the main passenger carrier. Airships were actually winning, because they were deemed to be safer than airplanes.
Airplanes weren't safe for crossing oceans before the Second World War, but airships had proven to be a disaster in terms of a military craft. They're not a very good aggressive craft, whereas airplanes were the best for that. So between the Germans, the Japanese, the Americans, the British, the French, and everybody else, in that five-year period half a million airplanes were built. We went from airplanes that weren't safe to cross oceans, to high-altitude bombers and jet engines. Then with the Cold War, they just continued to pour public money into investing in airplanes.
The private sector picked up on this, so the Boeing 707 aircraft came out as a passenger airline. Fuel was cheap. Nobody was carrying freight at that time, except in the bellies. Nobody cared about the environment. Every place you wanted to go there was a runway, so why would anybody invest in airships?
As time went on, even though people said maybe we should look at airships again, there was never a compelling case because airplanes had all the pilots and ground crews trained from the military. All the investment had been made in the aircraft already. There was really no compelling case to build airships. That is really why they have never gone forward. Now things have changed, and we're in a situation where they do make sense.