The main reason of ancient Greek immigrants is

Greece is not rich in natural resources, nor fertile and vast plains, only endless mountains. These mountains divided the land into small pieces, and the early Greeks settled in this isolated small piece of land and continued to expand. These settlements are generally called "city-states". The city-state is located in a place with relatively fertile soil or convenient transportation, thus attracting more immigrants and becoming a major city in the region. Many small city-states were formed in this way. They are isolated from each other and full of vitality. The famous city-states in ancient Greece were Athens and Sparta.