What two reasons did the spanish have for deciding not to focus on their northern borderlands

The northern borderlands of the Spanish colonies are now situated in the south of the United States. This place is rather dry and desertic compared with the centre of Mexico, what used to be the heart of the colonies. They didn't have the means to make it productive land and produce crops, and didn't have the workforce either. Indians living there were nomadic and offered great resistance to Spanish subjugation, the opposite from the tribes living in the centre of Mexico, sedentarian and already used to the dominance of an empire, the Aztec one.