The in-universe answer is that they didn't want to bring the Cylons to these lost straggler colonies. And their low tech meant they were apparently invisible to Cylons, since we don't see them wiped out by the Cylons though the Colonial fleet went by.
I can understand the idea of keeping the Colonials together, but they could improve both their fates by letting some Colonials settle on the backward worlds as long as they didn't develop the tech that was advanced enough for Cylons to show up and bomb it. The case of Terra proves it may be interstellar flight. Go set up power grids on the Western world and democracy on the prison planet.
And the Colonial fleet actually improves its odds for survival in the long-run, because they could force essential personnel to stay but let the emotionally strained or non-essential people leave. You extend the length of time your life support systems last, and you could bring the numbers down so you could crowd people into the remaining ships if one of them suffers a critical failure.