Here's why:
* Divine Wrath specifically targets spell and trap cards.
* Winged Dragon of Ra is a monster card, not a spell or trap card. Its effects are inherent to the monster card itself.
Therefore, Divine Wrath would have no effect on Winged Dragon of Ra.
Important Note: This doesn't mean Winged Dragon of Ra is unstoppable. It can still be destroyed by other cards, banished, or negated by effects that target monsters.