Question

Why motion of current is opposite direction of motion of electron

Answer 1

The direction of an electric current is by convention the direction in which a positive charge would move. Thus, thecurrent in the external circuit is directed away from the positive terminal and toward the negative terminal of the battery. Electronswould actually move through the wires in the opposite direction.

Answer 2

This is just a convention.
If positive goes forward, we just assume, for simplicity that negative moves backward.
That's all!
Don't dig deep into it. Its just a simple rule!
Hope it helped you!