Why is Poseidon called the father of commerce?

Poseidon is the god of the sea. And it is across the sea that the great ships have sailed for centuries to move goods and materials between ports. As just a single example, it is generally easier to move several cargo containers from an Italian port across the water to the Netherlands than to put those containers on a train and move them by rail. Certainly up until the 1800's, ships were the "best way to go" for moving lots of products between greatly removed sites around the globe. Many of the world's great cities are port cities, and for good reason.