There is not one but a few main reasons for using lead and concrete instead of particle deflection fields in a reactor shielding. First reason (and also the main) is that deflection fields act only on charged particles (alpha and beta radiation). A nuclear reactor usually generates neutron, X and gamma radiation all of which can not be stopped by an electric or magnetic field. Second reason is that the stopping power of a solid barrier (lead or concrete) is much bigger than the stopping power of a field barrier for electrically charged radiation. In other words, you need only a very small length of lead shielding when compared with field shielding. Third reason is that in all nuclear applications safety is a main concern. Concrete and lead stay in place, but what if the electric power supply of the deflection fields goes down? All the shielding is lost in this last case.