I just got done reading William Forstchen’s book, One Second After, about an EMP attack on the U.S. All electronics were wiped out taking the infrastructure with it. In the book, casualties were massive and the population was severely reduced a year later. In the BS:G finale, they took a spacefaring civilization and spread it out over a virgin planet teeming with new diseases and wild animals. This made me realize that without technology they would lose many people in the first year, and the original plan of building a central city was their best hope for survival and establishing a new civilization. Branching out and exploring could be done once basic survival needs were met.
Might explain why it took 150,000 years to reestablish things.