Wondering about Catarina crafting mods...

Why does the "+% spell damage" can't be applied to daggers but just to wands, staves and scepters when other mods can?

Just found a dagger with decent critical chance for spells but a low hybrid spell dmg + mana, but can't make it an useful item because of that "issue" (if it's an issue and not intended, otherwise I still can't understand the reason behind that choice).