The primary factor that kept ancient Greece from becoming united like the Roman Republic is b. geography.
The mountainous terrain of Greece led to the formation of independent city-states (poleis) that were often isolated from one another, which contributed to a lack of political unity. Each city-state developed its own government, culture, and way of life, leading to a diverse and fragmented political landscape. While culture, language, and religion were common among the Greek city-states, it was the geography that significantly hindered the possibility of creating a unified state like Rome.