Here's why:
* Crystal Beasts are monsters: Their main purpose is to be summoned to the field and attack, defend, or activate their effects. They are not designed to be used as Spell/Trap cards.
* The Spell & Trap Zone is for Spell and Trap cards: As the name suggests, this zone is specifically for Spell and Trap cards, which have their own unique effects and functions.
* "Crystal Beast" is a type, not a specific card: There are many different Crystal Beast monsters, each with their own unique effects and abilities. There are no Crystal Beast cards that are designed to be placed in the Spell & Trap Zone.
However, there are certain cards and strategies that involve moving Crystal Beast monsters to the Spell & Trap Zone:
* "Crystal Beast Rainbow Dragon" effect: This powerful monster can send itself to the Spell & Trap Zone to summon other Crystal Beasts from your hand.
* "Crystal Beast Sapphire Pegasus" effect: This monster allows you to remove a Crystal Beast from your Spell & Trap Zone to activate its effect.
* "Reinforcement of the Army" effect: This card can add a Crystal Beast from your deck to your hand by sending a Crystal Beast from your Spell & Trap Zone to the Graveyard.
In short, Crystal Beasts in the Spell & Trap Zone are inactive and unable to be used, except for specific card effects that allow them to be moved there temporarily for special purposes.