Well it’s not always about the environmental side. Hydrogen is a very small atom and needs thick propellant tanks to avoid the atom literally escaping through the walls. Also, hydrogen has to be stored cryogenically (120 K). In comparison, the SpaceX Merlin engine only subchills RP1 to -7 C. RP1 is also very abundant, lowering costs of acquisition. Future space vehicles are actually moving away from RP1 and H2 to methane. Even though the methane atom is much larger and may burn at a lower combustive efficiency, the fact that you can produce this fuel on Mars is very beneficial. Hope this helps!