Here's why:
* Spellbinding Circle's effect is triggered "when a creature you control deals combat damage to a player". This means the effect happens after the creature has already dealt damage, not during the attack.
* Attacks happen in the combat phase. During the combat phase, creatures are declared as attackers, blockers are declared, and damage is dealt.
* Spellbinding Circle's effect triggers in the damage step of the combat phase. This is after the creature has already attacked and dealt damage.
In summary: Spellbinding Circle's effect is triggered by combat damage, not by being attacked. You cannot use it to react to an attack.