Here's why:

* Flammability: Hydrogen is highly flammable and explosive, making it a significant safety risk. The infamous Hindenburg disaster in 1937, where a hydrogen-filled airship caught fire and exploded, is a stark reminder of the dangers associated with using hydrogen.

* Helium is safer: Helium is a non-flammable gas that provides the necessary lift for blimps. While it is more expensive than hydrogen, the safety benefits far outweigh the cost.

Modern blimps are filled with helium. This makes them much safer for passengers and the general public.