The tiny percentage that survived were forced to cannibalize their ships to create viable habitats, and by story's present date, over a hundred years later, very few signs of the original ships or their technological advancements remain. They originally planned to pass over Gunsmoke, as it was only barely habitable, but unfortunately one of the sapient genetically-engineered beings they were using as a power source chose that time to strike against humanity, causing the ships to crash on the planet's surface. Trigun: Project Seeds was a fleet of Generation Ships looking for a new planet after Earth got used up.
Either way, not only do their descendants' politics, economics and culture regress to match their pre-Industrial Revolution technology level, they also forget that their ancestors ever came from another planet, making the story at first glance seem to be set in a pure fantasy world.