Why are ionic solids non-conductive?
Because the ions that could could carry a current ARE NOT FREE TO MOVE in the solid state.
A solid ionic substance is composed of an infinite array of positive and negative ions held together by strong electrostatic forces. The ions are NOT free to move. Once these bonds have been disrupted, i.e. by dissolution in a polar solvent (water) or by pumping in enuff heat to melt the solid, the ions are free to move and carry a current.
This is something worth getting your head around.