Here's why:
* Icarus Attack is a Trap Card: Trap Cards can only be activated in response to a Spell or Trap Card effect.
* Normal Summon is not a Spell or Trap Card effect: Normal Summoning is a basic action in Yu-Gi-Oh! and is not considered an effect.
When can you activate Icarus Attack?
You can activate Icarus Attack in response to:
* Spell/Trap Cards: If your opponent activates a Spell or Trap Card, you can chain Icarus Attack.
* Monster Effects: If a monster effect is activated (such as a monster's ability to destroy another monster), you can chain Icarus Attack.
Example:
Your opponent activates "Mystical Space Typhoon" to destroy a Spell/Trap card you control. You can activate Icarus Attack in response to destroy their "Mystical Space Typhoon".
Let me know if you have any other Yu-Gi-Oh! questions!