Originally Posted by hedwig123

JKR explained this. It was because Voldemort gave Lily a choice, which he had never really done before. So even though, say, James also sacrificed himself, that didn't evoke a magical protection because Voldemort would have killed him anyway.
But Lily could have run and saved herself, which she refused to do, and so her choice to die to try to save Harry is what caused the protection.