Why cant positive charges be used to create current?

Actually, they can. In metals, it's common for an electrical current to be carried by electrons, and that's because electrons are light (low mass), and move around easily. But electrons are by no means the only charge carriers - for example, in a liquid, currents may be carried by positive ions; and in a semiconductor, either by electrons (which have a negative charge) or by holes (which have a positive charge).