Well, naturally they'd start the terraforming process on those worlds which were already most suitable for human life, so those rocks would get a head-start on infrastructure and development.
As the initial population began to expand, they'd need to push terraforming on less and less hospitable rocks. "Shindig" established that terraforming was ongoing during the series.
If one assumes that their particular terraforming techniques have an easier time heating a rock than cooling it, then it would make sense for the later-terraformed rocks to be further out from any star. And if one further assumes that some initially overlooked rocks actually had decent habitability due to proximity to secondary stars, that could explain why some outer planets seem to be fairly well-developed like Persephone.