I found what I believe is the BroJoeK source I mentioned. The Link to it is contained in this message.
Since this "triangle trade" involved a domestic leg, foreign vessels were excluded from it (under the 1817 law), except a few English ones that could substitute a Canadian port for a Northern U.S. one. And since it was subsidized by the U.S. government, it was going to continue to be the only game in town.Robert Greenhalgh Albion, in his laudatory history of the Port of New York, openly boasts of this selfish monopoly. "By creating a three-cornered trade in the 'cotton triangle,' New York dragged the commerce between the southern ports and Europe out of its normal course some two hundred miles to collect a heavy toll upon it. This trade might perfectly well have taken the form of direct shuttles between Charleston, Savannah, Mobile, or New Orleans on the one hand and Liverpool or Havre on the other, leaving New York far to one side had it not interfered in this way. To clinch this abnormal arrangement, moreover, New York developed the coastal packet lines without which it would have been extremely difficult to make the east-bound trips of the ocean packets profitable."[2]
Even when the Southern cotton bound for Europe didn't put in at the wharves of Sandy Hook or the East River, unloading and reloading, the combined income from interests, commissions, freight, insurance, and other profits took perhaps 40 cents into New York of every dollar paid for southern cotton.
So you would have us believe that U.S. ships from Europe, crammed to the gunwales with imported goods destined for Southern customers, would leave England, dock in New York, unload the goods, pay the tariffs, load the goods back onboard again, sail down to Charleston, unload the goods, load with cotton, and then sail back to England? Really?
You left out this sentence: "As for the cotton ports themselves, they did not crave enough imports to justify packet lines until 1851, when New Orleans hosted one sailing to Liverpool."
The consumer demand just wasn't there to support much direct trade between Southern cities and Britain or France -- or rather, trade became more profitable when Northern cities were included in the loop.
The trade went through New York because Northern cities supplied by New York had a great desire for imported consumer goods. As rich as slave owning planters were, the density of prosperous free citizens was greater in the Northeast.
Robert Greenhalgh Albion sounds like a man after your own heart: somebody who stresses the mercenary, cutthroat aspect of commerce and takes pride in the successful exercise of cunning. I would look for rational reasons why New York predominated as a port, and one important reason was the South's lack of interest in seafaring.
Another reason was, as noted, the greater population density in the Northeast, and also the rail and canal connections between New York and other cities. Indeed, once you have many people in banking, finance, insurance, shipping and related fields in a city they attract business.