Human/Undead Cold SLOW Damage comparison

did anyone wonder why
-Frosty can have a cold damage because it breath cold damage and slow enemy unit ?
-frost nova can have cold damage and slow the enemy unit
-but WHY not Blizzard skills that hit rain the ice shards does not have cold effect? this need to be fixed. cold damage slow from blizzard skill should be applied.

PLEASE BLIZZARD, make BLIZZARD a real ice slow enemy unit like frost wyrm or frost nova that able to do.