Because a roll is not a turn. You can get a standard rate turn with just rudder, if you want. A good way to figure this out is set a jail-breaked turn coordinator out on the desk on a flat surface, align it facing you, and push the end of the case in the direction you want to "turn" (so the case twists), then, from where you pushed to achieve this, follow the gyro 90 degrees in it's plane along the direction of rotation and push there (where the force is felt). You'll see the gyro tilts and causes the indication.