Hydrogen the atom is very common but what is needed is hydrogen the gas i.e. molecular hydrogen or H2. Hydrogen is very reactive so it's usually always bound up, for example in water. Stripping off the hydrogen atoms into H2 gas is energy intensive.

Since it's so reactive, it's both a pain to handle and to extract unless you have a lot of natural gas reserves, where it's relatively abundant.

Or you find it in the interstellar medium but I mean good luck mining a nebula.