Ghost type? Hail lets ghost type hit normal, or boosts their Def or SpDef? Ghosts are associated with cold quite often.
What if Hail slowed fire-types? The cold is getting to them?
Oooh I got it! Fighting types! They ignore hail and it boosts their Def, because fighters training on the top of mountains and in the cold and breaking ice is canon. Creates nice symmetry with water/electric boosted by rain and fire/grass boosted by sun, both the type and its weakness get a boost.