Here's why:
* Mystical Space Typhoon is a Continuous Trap card.
* Continuous Traps are placed face-down in your Spell & Trap Zone and activate during your opponent's turn.
* You cannot activate a Continuous Trap card during your own turn.
However, you can activate Mystical Space Typhoon from your hand using the following:
* "Harpie's Feather Duster" - This Spell card lets you destroy all Trap cards on the field.
* "Twin Twisters" - This Spell card lets you destroy 1 Spell/Trap card on the field, then you can banish 1 Spell/Trap card from your hand.
Remember, Mystical Space Typhoon is a powerful card for removing your opponent's Spell/Trap cards, but you have to use it strategically during their turn!