Originally called Lawman, Call of Juarez was initially conceived as a reaction to World War II and science fiction games dominating the first-person shooter genre. The designers drew inspiration from a range of Western-themed films, TV shows, and literature, as well as some real-life stories and figures.
It's the curse supposedly placed by the Aztec Sun God, Huitzilopochtli, upon the gold used as the ransom for Montezuma when he was held captive in Tenochtitlan. Juarez is the name of the border town where this lost gold is rumored to be hidden, and that's how the curse came to be currently called.