Before railroads, the Erie Canal was one of the few ways to ship bulk goods over the Appalachian divide. This caused the mouth of the Hudson to be a center of trade, particularly shipping goods to Europe. To make money on the return trip, they would put passengers in steerage and so lots of immigrants landed in New York City.
Even before the Erie Canal, New York City was basically the best port on the Eastern seaboard so it was a natural trading center when it was initially settled by the Dutch. (And the Hudson provided good access to the interior even without the canal.)
