Why did many greek city-states encourage migration to areas outside of Greece during the 8th and 7th centuries BC?

Many Greek city-states did not encourage migration outside of Greece during the 8th and 7th centuries BC. In fact, some even discouraged it by imposing heavy sanctions on emigrants. However, some city-states such as Miletus did encourage it because of population pressures (over population within their small state), land shortages from a scarcity of arable land resources or political conflict and civil strife due to a rising merchant elite within the Greek city-state which had become more interested in overseas trade opportunities