I expect Earth would have broken up into competitive states/regions after the great burn while people fought over limited and safe resources. The colonies, especially the closest on Mars, may have not wanted to get involved in a cluster fuck like that. Mars and other colonies may have long since gone independent of Earth that making Earth go so xenophobic there was a planet blasting war. Off world humans may have gone alien so to speak and given up dealing with Earth's imperial ambitions and pushiness deciding after the great burn to leave them to their misery while the colonies take advantage of their place in the Interstellar Alliance. The quiet moving Rangers are like UN advisors and aid of sorts. They aren't there to fix things but to help the folks on Earth fix their problems more of which are likely social ones that are far harder to fix. Dumping technology on Earth in its blighted condition might ignite more problems than it would fix.