They believe the traveller will bring about their death. Hence the dead orbit name. Their name implies the traveller, being in a geosynchronous orbit (meaning it's position relative to the city on earth doesn't change), brings death. Also, according to wiki, they believe the City won't hold against a major attack. [spoiler]it didn't. You know, the first thing that happens in the game.[/spoiler] They believe in leaving earth for humanity's continued existence.