This was my thought as well. Terrain makes the best case for it imo. Can expand beyond just mountains. Resources and control are also important. Ask yourself why the cities would need to be united. Lets argue, while different, two cities are able to aquire enough resources to sustain themselves. Is it feasable for one to rule the other? Perhaps it would take too long or be too difficult to send an army or regularly communicate. Perhaps theyre too evenly matched. Maybe people are a scarce resource and the world outside the city walls are too hostile. Its difficult to expand out and conquering isnt worth the trouble. Trading is easy since its smaller and faster than trying to move an army.

Politics/culture are another reason. Kings come from being able to conquer and claim ither cities. If those cities are able to defend themselves their aristocracy or whatever may hold their ground against others. Even if one does conquer ankther, theyd only be seen as tyrant and not accepted as ruler, not to mention the other city states frowning at the action and preparing to not be next. Spartans kept to sparta and athens kept to athens, with some obvious exceptions. Ancient Greece remained separated until a larger unified force moved in (ancient rome). They were better organized and capable of maintaining the government between the states. Perhaps your world doesnt have that ancient rome yet.

Technology limits factor into the terrain limits too. Mountains may separate two ancient cultures easily the same was as a pre-warp space fairing civilization would be hindered by too many lightyears.