Here's why:
* Blackwing - Gale the Whirlwind's effect reads: "During your opponent's turn, if this card is in your hand: You can send this card to the Graveyard; Special Summon 1 "Blackwing" monster from your hand or Deck."
* "During your opponent's turn" is a crucial part of the effect's activation condition. It means the effect can only be activated during your opponent's turn, not your own.
Therefore, you can only activate Blackwing - Gale the Whirlwind's effect during your opponent's turn, not your own.