I have not read Templar One at all, but old prime fiction stated that they had a "short dark age". Which, admittedly, could be something like having a fully functional colony and then figuring out how to keep things in good repair after a few generations.
E.g. (Mith's musings here, not canon)
- Amarr had high tech, but their colony was incomplete and/or depended on others for maintenance. When interstellar travel stopped, they became preindustrial once the advanced tech stopped to work. Probably some of the old knowledge exists in ancient Scripture: How much of this is understood (some may be rewritten) is debatable. There may have been some kind of an "Encyclopedic" focus.
- The initial Caldari were the technicians etc. who were on-planet to keep an eye on the terraforming process. While losing capability to travel in the stars, the original settlers could sufficiently over-engineer the terraforming equipment such that future generations of proto-Caldari could live in habitats until the planet was sufficiently terraformed. However, the ability to repair the machines made by the Ancestors was lost eventually, and after the planet was sufficiently terraformed that the Caldari were no longer dependant on the terraforming equipment, lost altogether. (Later, with Gallentean help, some advanced technology was recovered from the ruins.)
- The Gallente and Minmatar had a similar fate than the Amarr, though their planets were slightly more suitable for human life and had colonists of different culture. Can't remember details about Gallente history on the top of my head, but if I am not terribly wrong, Minmatar were the first to form a World Goverment after the collapse - they just did not for some reason rediscover near-lightspeed or faster-than-light travel.