Maybe because there’s no fleet of airships, and they would make poor water delivery vehicles. They’re slow, for one thing. The Hindenburg could lift 10,000 kgs in addition to its own weight. That’s about 2,600 gallons of water. An Olympic-size swimming pool holds 500,000 gallons for comparison. A C-130 supertanker can carry 19,600 gallons, 7.5 times a huge airship’s lifting capacity, and can get to the fire and back to refill a lot faster.
This might make a good tech interview question for people who already know how many piano tuners work in Chicago.