The Saturn V only used hydrogen for its third stage. The first two stages (i.e. the ones that operated while it was still in the atmosphere) ran on RP-1.
Hydrogen offers excellent ISP but poor volumetric energy density, and it needs to stay much colder than oxygen to remain liquid. Fuel tanks for containing hydrogen tend to be more complicated and larger than those for other liquid rocket fuels. Upper stages see more relative benefit from the higher performance achievable with hydrogen fuel. The cost:benefit comparison has led to most rockets not using hydrogen for the first stage.
"The cost:benefit comparison has led to most rockets not using hydrogen for the first stage."
nobody factored in the long term climate cost with kerosine. But since most hydrogen is generated with fossils, it might not make a big difference right now.
Hydrogen yields higher efficiency (specific impulse) with lower thrust. Larger-molecule fuels, such as kerosene, have higher thrust and lower efficiency. When in the atmosphere, the rocket engines are spending a lot of energy counteracting gravity, so higher thrust is important. Once in orbit, efficiency is far more important (indeed, some deep space engines such as ion engines produce only millinewtons of force, but are very efficient.)
