I think, based on the entries for Glisten and Bowman, the two Ateroid Belt systems out campaign deals with most frequently, that if the 'main world' in a system is rolled as Asteroid belt, then that asteroid belt takes the place of every planet in the system and all places in that system are on asteroids of one sort or another.
For actual planets, I could totally see the owners of land on the nice planets in a system charging the starport developers way too much for the land and water rights they'd need, and the Starport people just moving to a cheaper location rather than pay through the nose for services.