Because linear velocity and angular velocity (the speed at which an object spins) are two different quantities, as is already apparent from the units (m/s and rad/s). Saying that something rotates with 1/4 c is simply not a useful statement. It would be useful if you were saying that, for example, an object at the event horizon travels at that speed. But that speed is dependent on your altitude above the massive body. The same angular velocity (rotational speed) at greater heights translates to lower speeds than further below.
Because linear velocity and angular velocity (the speed at which an object spins) are two different quantities, as is already apparent from the units (m/s and rad/s). Saying that something rotates with 1/4 c is simply not a useful statement. It would be useful if you were saying that, for example, an object at the event horizon travels at that speed. But that speed is dependent on your altitude above the massive body. The same angular velocity (rotational speed) at greater heights translates to lower speeds than further below.