Why is it said that electric current always flow from higher potential to lower potential?

What do you mean by positive electrons? There are no positive electrons. Once an electron is lost from an atom, it's an ion, not a positive electron. The ions cannot move because they are tightly packed. It's the electrons that are free to move.
Now the flow of electron from positive terminal to negative terminal is related to conservation of energy. The electric source (a battery probably) is a source of electric energy. That energy can be absorbed only by charged particles. So the electrons get excited by this energy which makes them to move around.
You may have studied that energy flows from a higher concentration region to lower concentration of energy. A battery's positive terminal holds higher energy than the negative terminal.