Question

Why on the conductor electric field is zero?

Answer 1

An electric field is caused, on a physical level, by the fact that you have more electrons on in one point in space than in another. The difference in charge causes the field. However, the field exerts a force on the electrons to make them move. If there is no resistance, i.e., if you have a conductor, the electrons are free to move and equalize in space. As such, the field drops to zero.