Lighter than air, and too slow
Why Did Airships Stop Flying?
An airship floats without moving and needs almost no fuel to stay up. It disappeared anyway, and fire is only part of the reason.
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Step 1 of 5
Anything lighter than the air it displaces floats.
A balloon rises for the same reason a cork rises in water. Push the air aside with something lighter and the surrounding air pushes it upward.
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Step 2 of 5
Which means it needs an enormous volume.
Air is very light, so lifting anything useful means displacing a great deal of it. An airship carrying a few dozen people has to be longer than a football pitch.
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Step 3 of 5
Hydrogen lifts best and burns.
The lightest usable gas is hydrogen, and it catches fire readily. Helium is safe and lifts nearly as well, but it was scarce and controlled by one country at the time.
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Step 4 of 5
Size is the real problem.
Something that big is at the mercy of the wind, needs a crowd of people to moor it, and cannot be flown through weather an aeroplane simply climbs above.
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Step 5 of 5
Aeroplanes got fast, and that settled it.
An airship crossed the Atlantic in two or three days. Once aeroplanes could do it in hours, floating gently stopped being worth anything to a passenger.
A handful still fly, mostly as camera platforms over sports events, where hovering quietly for hours is exactly the point.
The short version
An airship floats by weighing less than the air it displaces, but its size made it slow and weather-bound, and aeroplanes won.
Try it yourself
Let a helium balloon go in a room with a draught. It has no way to resist being pushed around.