Here's why:
* Mystical Space Typhoon targets Spells and Traps. It specifically states "Destroy 1 Spell/Trap Card on the field."
* Equip Spells are treated as continuous Spells. While they are attached to monsters, they are still considered Spell Cards.
* Mystical Space Typhoon cannot target monsters. It is designed to deal with Spells and Traps, not monsters.
To get rid of an equipped monster, you would need to use a card that targets monsters, such as:
* Monster Removal: Cards that directly destroy monsters, like "Raigeki" or "Torrential Tribute."
* Effect Negation: Cards that negate a monster's effect, like "Solemn Judgment" or "Effect Veiler."
* Tribute Summon: Cards that require tributes to summon monsters, like "Tribute to the Doomed."
Remember, card interactions in Yu-Gi-Oh! are based on specific wording and effects. Always check the card text to see what it does and how it interacts with other cards on the field.