Castilian was not transfered to the new world because it was predominant==
It was just because the conquest of the new world was made only by Castilla, and only Castilians paid it, while Aragón was involved in its own wars in the Mediterranean. Even in Flanders, you can see which Kingdom the troops were coming form, because until the Bourbons took over, there were internal borders, taxes, different armies, currencies, courts, &c.;

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