Here's why:
* Mystical Space Typhoon's Effect: Mystical Space Typhoon allows you to destroy a spell/trap card on the field.
* Activation vs. Destruction: Activating a spell/trap card means putting it into play. Destroying a spell/trap card removes it from the field entirely.
Therefore, Mystical Space Typhoon destroys a spell/trap card after it has been activated. It does not stop the card from activating in the first place.
Example:
1. Your opponent activates "Torrential Tribute," a trap card.
2. You activate Mystical Space Typhoon and target "Torrential Tribute."
3. "Torrential Tribute" resolves and destroys all monsters on the field.
4. "Torrential Tribute" is then destroyed by Mystical Space Typhoon.
Even though you destroyed "Torrential Tribute" with Mystical Space Typhoon, its effect still resolved and destroyed your monsters because it activated before being destroyed.