Here's why:
* Divine Wrath states: "When a monster is Special Summoned, destroy it."
* Special Summoning due to an effect means a card's effect is used to put a monster on the field.
* Divine Wrath only activates when a monster is Special Summoned. It doesn't care how the monster is Special Summoned (by effect, from the hand, etc.).
* However, Divine Wrath cannot be activated when a monster is Special Summoned by an effect if that monster's own effect is what Special Summoned it.
In the case of Cyber Dragon, if it is Special Summoned by its own effect (such as "Cyber Dragon Infinity"), you cannot activate Divine Wrath.
Example:
1. You control "Cyber Dragon" in your hand.
2. You activate "Cyber Dragon Infinity"'s effect to Special Summon "Cyber Dragon" from your hand.
3. You cannot activate Divine Wrath because "Cyber Dragon" was Special Summoned by its own effect.
Conclusion: You can only activate Divine Wrath when a monster is Special Summoned, but not if that monster was Special Summoned by its own effect.