Here's why:
* Obelisk's effect is a continuous effect. Continuous effects are ongoing and cannot be negated by cards like Divine Wrath, which only negate activation effects.
* Divine Wrath only negates the activation of a Spell/Trap card. Obelisk's effect is not activated, it's a continuous effect that's always in play.
Therefore, Divine Wrath cannot negate Obelisk the Tormentor's continuous effect.