Hello,

This is basically can be explained on the basis of gauss law, and  according to this statement it simply stating that the the intensity of electric field on a surface of a conducting or non conducting sphere  depends upon the intensity of the electric field on that surface and the no. Of electric field of lines passing through the surface. That is why the electric field due to discrete charge configuration is not defined at the locations of the discrete charges , because it all depends upon the no. Of electric field lines and there directions and thier intensity.

Hope this helps!

Good luck.