They were selected because of the protection they provided from the sea without the need to construct any defences (which they would not have had the technology to build). Can access to and from the Mediterranean sea can be blocked in the Straits of Gibraltar.
Europe has a long-running history of shipping rolling back to the Vikings era (793 AD onwards). The start of ship trading saw the wooden ships with wind sails explore the unknown world.
However, the discovery of the merchant trading possibilities promotes business at the 10 biggest ports in Europe.