The consolidation of power is basically an unstoppable contagion. Once more and more states start creating strong, centralised nations, it becomes more and more difficult for remaining city states to remain competitive.

Similarly, once a city state is conquered, it enters a larger state or empire. Even if that regime is overthrown by foreign invaders or interior usurpers, the city state will likely remain under exterior rule.

Finally, city states aren't really sustainable. When other international players can draw on huge armies dispersed across a larger area (see Alexander the Great or the Romans), it's unimaginably difficult for city states to draw up even remotely comparable forces, like Sparta and the 300 at Thermopylae. The same is true of resources.

I'd say that city states' success in remaining historically relevant was far less economic or military than it was cultural. Athens was certainly affluent, but in a Qatar or Monaco sort of way—famed for its wealth and likely to be able to hold its own in tactically favourable conditions, but really too small to ever cause much trouble. What we really remember it for is the philosophy, the democratic innovations, the art and architecture, etc.

Years of geopolitical military manoeuvring has made it practically impossible to succeed outside of a centralised, larger state.

Not really an expert, but thought this might help.