Seems like for the multibillions of dollars spent, they could have thrown in a stepper motor. All it had to do was spin around and take a photo every few degrees.
All thing are more expensive and more complicated in space.
The whole assembly has to work at -200 degrees below zero. And survive radiation. And be tested over and over. You're talking millions more dollars just for a rotating camera (and again remember this isn't the digital cameras of today; it's the digital cameras of 15-20 years ago.
They possibly would have had to remove another science instrument due to budget or weight constraints.
It's not the money, it's the weight. As it was, the probe was so heavy that they couldn't just launch it at Saturn, they had to set it up for a bunch of fly-bys of Earth, Jupiter, and (I seem to recall, I could be wrong) Venus and take 7 years to get to where they were going. The weight of a stepper motor, a bearing, a shaft and mount for the camera, etc., etc., might have caused the elimination of another instrument.