The major lore reason is likely the fact this is a fantasy game with feudal governments and there is no middle class. Hyur are the most common of all the races so they're more likely to be poor as opposed to other races who often have niche jobs they can do well.

From what I can tell Miqo'te almost never really use Gil unless they're either born outside the clans, left to live among the other races or go adventuring. They merely trade for the things they need or make/hunt it themselves.