I suspect you're probably right. She's a very effective "behind the scenes" person and only comes center when she's asked. When she does come center, there's no mistaking her capability and character.
She's another unsung hero of this campaign, there's many, many more like her.
Besides, doesn't Karen Hughes write many of his speeches in collaboration with others? For his approval and editing, of course, but that might be the point at which she asked not to be mentioned.