Next to the comment above, I would guess that the fact that all instrument procedures are based on rate one turns has something to do with it. If the only situation where a rate two and above turn is needed is during training, then the addition of these markings to a TC or T&B is a bit of a luxury. Admittedly training a steep turn on a AI is slightly different from training this on a T&B as you're aiming for an angle of bank on the AI while it is a constant yaw rate on the T&B, but I wouldn't be bothered by the difference. Actually, now that I'm thinking about it, at rate four the vertical axis of the airplane and the axis about which the aircraft is turning are so misaligned in roll that you could argue that it wouldn't be accurate anyway.