This actually happened to a few cultures over history. What tends to happen is that when a culture becomes highly advanced in relation to it's neighbors it can enter a very self-indulgent period of introspection and navel gazing.
Or to put it a better way, their advanced state leaves them disinterested with outsiders. Not xenophobic, just indifferent. China, for example, became so advanced that she was content within her boarders, engaging in philosophical and artistic pursuits.
To make this practical in your world, the outliers would need to be beyond your advanced nation's secure borders and pose no threat. The nation would need to be wealthy and self-sufficient, I.E. have plenty of resources within it's boarders and trade for what it considers luxuries and not necessities.
To prevent a spill-over into your border villages, have them trade with the villages for luxuries, not necessities. The villages would become specialized in making those luxuries and would not be geared towards production of necessities, thus stunting their growth.