True, hydrogen can't be ruled out as a fuel source. Brotherhood used it for their zeppelins. Someone could figure out how to power cars with it.
My point is why? They have cars that run on MF cells and Cars that run on bio-fuel. Before the Great War MF Cell cars were the tops. Electric and therefore environmentally fiendly. No need to worry about Gas anymore. Still Uranium was also running out and MF was still a new tech, which is why there arn't that many different Energy Weapons.
brotherhood zeppelins didnt use hydrogen for a fuel source, but as a lighter than air inflatant- like they did in our universe as well as the fallout universe in the 1930's (before the divergence). The hydrogen bomb also was just that. a gaint fuel source.. only instead of attaching a load that carries out work, via steam power, the energy is dissipated into the atmosphere.
and between the 1930's and 2077, i should think Hydrogen was used for automobiles. and, they were:
http://fallout.wikia.com/wiki/Oil read under cheaper fuel.
The answer to your question of "why?" because, everything was running out.. why wouldnt ya SCIENCE! minded community not?
You are aware that MF cells are not batteries, but are nuclear reactors, right? niether here or there. but..