Here's why:

* Metallic Structure: Metals have a unique structure where their outermost electrons are loosely bound to the atoms. These electrons are free to move throughout the metal's lattice structure.

* Electric Field: When an electric field is applied across a metal, these free electrons experience a force and begin to drift in a specific direction, creating an electric current.

* Drift Velocity: The electrons do not move at the speed of light, but rather drift slowly in the direction of the electric field. This slow, directed motion of electrons constitutes the current.

Important Note: While protons are also present in the metal's nucleus, they are much heavier and more tightly bound, making them essentially immobile. Therefore, they do not contribute to the electric current.