Why does gold dissolve in mixed acids and not a single acid?

You are talking about a matter of degree. Gold is soluble in water without the acid, but not much. Ocean water often contains maybe one part per million of gold. Natural streams are acidic because the rain picks up carbon dioxide, and they contain nitrates because the rain falls on fertile soil. So any stream through gold country dissolves enough gold for miners to consider it a nuisance, and they buy mercury to take the gold out of the water. (They shouldn't, but they do.) Aqua regia dissolves a lot more than other solutions, but that is not to say that solubility in other solutions can be ignored.