The law of economics, to which US law must also bow. The Navigation act of 1817 made it uneconomical for them (foreign ships) to visit Southern ports, because they could only visit one. (for profit.)
So the navigation act of 1817 was the only reason in the world, that Europeans shipped to New York or other Northern port. Has nothing to do with the fact that New York had more capacity for ships than all Southern ports combined, that it was two day closer sailing. none of these factors applied, only the Navigation act of 1817. So the millions of bales of cotton that left New Orleans, Mobile, Charleston, Savannah between 1817 and 1860, were all hauled out by Northern ships.