"some hundreds of leagues out of the ordinary course of the trade of mankind"
Triangle Trade Routes
Creative Commons Attribution Share AlikeTriangle Trade Routes - Credit: SimonP, Wikimedia

At this time, Atlantic trade generally operated along a standard ‘triangle’ route, to make the most of the prevailing winds and serve the demands of the different markets.

Slaves were taken from Africa across to the plantations in the Americas, where sugar and tobacco were picked up and transported to Europe. From there, rum and textiles were transported to Africa, where more slaves were collected.

Somewhere off South America, Crusoe is not on the standard trading routes and so has little chance of being picked up.