We can look on Rhul-Thaun and isolationism like on population closed in Vaults in Fallout universum. In many vaults attempts to interact with the outside world ended badly. New mutated species, saved new civilizations and cultures etc. was destructive for “population from old-world”. Even in situation, that new populations were most primitive, in compare to Rhul-Thaun, they are better adapted to new rules of new world.
Ofc. In Fallout universum many new cultures had own origin from Vaults, and there was connected with experiments. But for topic problem very important element should be fact, that almost all trials opening to external world was failed. And even old, better technology, wasn’t sufficient advantage to psionic, arcane magic, number, knowledge of the environment etc. etc. Firsts confrontation wasn’t entirely destructive, but so severe that the terrified ancient halflings refuse to explore.
Other concept can be located in radical fear. We can look on few historical population from period “Collapse Bronze Age” or Mayans after first collapse their civilization. Saved people believed that the outside world had ended, that dragons, monsters, or darkness ruled there. Eg. in classical hellenic age still were small populations of survivors from minoan civilization, who have believe that external world was destroyed by earthquakes, tsunamis, volcanoes etc.