Because it is stable for an explosive element, can be pressurized into liquid form which can save valuable space, and is reactive enough to provide excellent thrust.
Liquid hydrogen contains a lot of energy in an extremely small space. A rocket needs to carry the weight of the fuel as well as the payload. As a result, the United States uses liquid Hydrogen for...
Hydrogen basically means "water maker" in Greek, because it forms water when burned."Hydro" means water, "gen" means generate. When hydrogen is burned, it produces water (vapor).