Why is Titan the only moon with a substantial atmosphere?

Why does Ganymede have a global magnetic field, while so many other large, similarly massed moons don't?

This seems rather inefficient. Why not focus on electrical storage or generation and use the electricity you would have used to produce the hydrogen instead for direct heating? Hydrogen is such a pain to work with and leaks out of literally every material known to man. I Feel like the only advantage hydrogen fuel ever has is energy density, but that usually is only valuable for vehicles that would struggle to make the switch to electric due to the poor energy density of batteries.