I think one of the main reasons this cleaner was not great seller in the UK was due to the fact that not many retailers (be they physical shops or mail-order) chose to stock this particular model.
As to why it was not stocked, there are several possibilities. Bagless cleaners were not really as sought after then as they later became, because there were still a number of cleaners which could have cloth bags instead of paper, particularity in the cylinder market.
Another issue is the cleaner itself. Although by 1984 it was the most modern of all the Senior cleaners which had gone before it, the style was actually very dated and somewhat utilitarian in design. Had this cleaner been sold to the commercial market, we may have seen more of them.
Had the dustcup been fitted to the Turbopower cleaners (which, at that time were state of the art for Hoover), we may again have seen more examples. But the dustcup was a "new" idea for the UK and it was attached to an old model, and instead of reviving an old model and making it more modern, the reverse seemed to occur and a new idea was overshadowed by the dated cleaner it was attached to.
As well as that, it would have been confusing for consumers to see that the flagship Turbopower relied only on a paper bag, giving way to the idea that if they weren't bagless, was a dustcup a truly beneficial option in general?
By the 1990's, many consumers were getting fed up with paper bags, and the success of the permabag 'Freedom' range of Hoover Turbopower and Turbomaster cleaners shows what can happen if the right cleaners have the right features and are advertised accordingly at the right time. The failure of the permabag in this example is of course another story.