I think it was due to cost, Steve. Hoover knew that by the time the junior and Senior were not making money, they had to act fast! It would make sense to re-body a similar elderly component that had done before and put a new plastic design around it. The TP series may have been dirty fan vacs but buyers weren't exactly told about that. I can remember back in the day, terms like "dirty fan," and "clean air," just didn't float around.
Buyers were demanding lighter uprights and Hoover responded with the Turbopower uprights. They also went to town with their cylinder vacs, adding gadgets, lights and much more power - so there would always be a good compromise over a cylinder vac or upright.
Having said that, my parents never bought into the Turbopower dream. They had a Senior Ranger and then the Junior for most of my early years. Clearly they were into longevity and robustness, finally going with Oreck as the Junior's replacement, which was far too short lived and then replaced with the Sebo X1 Automatic.