1) The Fallen Empire
Once upon a long time ago, the whole planet spanned numerous grand, technologically advanced nations (or one Super-duper Empire). For unknown reasons, they declined, their grandiose technology forgotten. Only the most backwater university city and its surrounding cities managed to retain most of their technology. Fearing that history will repeat itself, this time with them at the center, they remain behind in their cities, mostly closed off to the world.
2) Deviant Tribe
This particular tribe of humans encountered a genetic mutation (or whatever) early in its history. This made them highly intelligent, but less-resistant to what nature could throw at them. They have low birth rates and an isolationist, peaceful nature. Few traveled outside their own region and those who returned told tales about the harshness of the outside world. They decided that surrounding themselves with high techs and comfort should be the priority and left the exploration of unknown to their machines.
3) They built BIG WALLS
Surrounded by aggressive, barbaric tribes, they build big, big, beautiful walls and focused on advancing their society to counter the threat, which would one day surely overcome their WALLS. They got so immersed in this undertaking, they didn't notice that they vastly overshadowed everyone else and has nothing to fear. However, their ingrained fears of the outside and the feeling of security associated with the big, beautiful walls prevented them from taking over the world. They opted for playing video games, in their endless virtual worlds, instead of dealing with the mundane, boring outside.
These a few archetypes, which you can reform in whatever way you want. There must be others out there too. Morris The Cat mentioned Wakanda, which was the Deviant Tribe version with the unique resource spin. Atlantis, would be the 3, with the geography of surrounded by oceans and so on.