Because initially the safety measures of the vehicle was designed for fuel, now converted to CNG.
In term of safety CNG is more safer than any of the alternative. CNG combustion produces less air pollution than gasoline, and compressed natural gas is also safer than other fuels in the event of a spill, because natural gas is lighter than air and disperses quickly when released.
As taught in chemistry class, I know how flammable natural gas is. Users should use a very thicken storage or tanker to avoid ignition. CNG is for factories or demands with no access to pipeline networks because they are transported by CNG tubes which are highly inflamable. The higher the distance the higher the rate.