The period between achieving space flight and the unification of Sol was not kind to the birthplace of Humanity. Earth is now a desolate, war-torn shell, populated by teeming billions of poor and downtrodden people unable to afford the excessive cost of getting into orbit and into the rest of the system. Earth is fiercely independent of the Solar government and rejects the authority of Mars, serving the Lunar corporations as a kind of tax haven and conglomerate of city-states.
By contrast to her sovereign planet, Luna is a gleaming jewel. Vast quantities of political/economic power and money reside in the cities constructed all across the Lunar surface. Corporate power on the moon easily outclasses the government, making it a playground for the hyper-rich and not an easy place to live for anyone else. Luna also administrates travel to and from the desolate surface of Earth.