why do only moving particles experiences magnetic force?

This is because in the moving particles, the magnetic domains in the molecular level are aligned giving rise to magnetism which is not the case when they are at rest

In relativity theory, there is essentially only one force; Einstein wrote "...What led me more or less directly to the special theory of relativity was the conviction that the electromotive force acting on a body in motion in a magnetic field was nothing else but an electric field."