Question

Why dont we use hydrogen as a common fuel in our automoboiles?

Answer 1

There are several problems with a hydrogen-based fuel engine. Surely, it is an eco-friendly way to ride a car but there are many problems associated with it. Some of them are:
Extraction
The only economically feasible way to get hydrogen gas right now is via fossil fuels. It is produced by a process called steam reforming which uses methane (natural gas) and steam. It doesn't make sense to use hydrogen from fossil fuels when we can easily run using the fossil fuel itself. There is no other cheaper way available. Electrolysis has been proposed but it takes so much electricity to do so that we're better off using that electricity to power our vehicles.

Storage
Hydrogen is a gas and the molecule is very small indeed. It can slip past small holes and orifices easily. So, it is important to make a vessel that has no such defects. In addition to that, the hydrogen needs to be stored under huge pressure. Hydrogen gas can also embrittle many metals, so a tougher material must be used. This adds to the cost of storing hydrogen, making it seem less feasible. Hydrogen is also extremely flammable, so care must be taken so it does not ignite.