why is hydrogen fuel cells not mandatory in all new vehicles?

surely this would make sense as the hydrogen fuel cell creates only water vapor as its waste product and the production of hydrogen costs no more then production of petrol/gas via the refinement of crude oil. As such it would cost no more to fuel you car with hydrogen then it would to fuel it with petrol/gas.

Furthermore, the oil production companies would not be losing out if they changed from the refinement of crude oil to the production of hydrogen. the Fuel companies would not lose out either as hydrogen costs the same as petrol/gas.