Why is Persephone called She who should not be named ? Hades name can be spoken but why not her name ???

Persephone isn’t necessarily called “She who should not be named”.  She is called “The Destroyer”, and depending on interpretation, she is the destroyer of light, or destroyer of the voices of men.  She had several epithets so that people would not invoke the Destroyer, such as Savior (Sotiera), the best of the underworld gods (Aristi Chthonia), the Bringer of Fruit/Spring (Karpophoros).

Hades (Aides, Aidoneus) translates to the Unseen One, or the Unseen Lord, and his epithets were many.  He was the Receiver of Many (Polydegmon/Polydektes), the Host of Many (Polysemantor), the Rich One (Plouton), the Good Counsellor (Euboleus, an epithet also applied to several other deities), the Notorious One (Klymenos), etc.  Actually, if anyone’s name couldn’t be spoken aloud, it was his.

Most of the Chthonic deities had epithets to dissuade people from calling on them directly.  The Fates were referred to as the Sparing Ones.  The Erinyes were called the Kindly Ones.