Hmmmm, let me share my thoughts.
Dyson unveiled their clean-air bagless models and they surely changed the market. It was much easier to imitate their design with a clean-air base. I know Hoover made their direct-air twin-chamber models, but boy did they leak and they did not work all that well once the filter became dirty. Cheap direct-air machines likely fell our of favor because companies such as Bissell and Dirt Devil began offering extremely cheap clean-air models that had on-board attachments. To make the cheapest machine possible, most companies looked overseas and from that came way more clean-air machines than direct-air machines. Certain companies also began advertising how their clean-air machines could suck up anything, building off the fear that a good majority of customers had about sucking bigger debris into their machines. I think a lot of factors played into it.
If you do it right, a clean-air machine can be a far easier to use machine just as a whole. Many direct-air machines clean carpet excellently, but flop in attachment mode. For example, the Hoover Elites cleaned great in upright mode, but are not all that useful for anything much more than dusting and getting in corners in attachment mode. I have seen numerous different tries, none of which have floored me (no pun intended).
Some of the best ones to use in attachment mode are the G-Series Kirbys, but transitioning to attachment mode is a little annoying and they still do not work nearly as well as something such as a $130 Hoover Tempo upright or a $250 Kenmore Elite upright. Overall, while it is a great machine, their design is quite dated and is not appealing to the average consumer.
I just rebuilt one of the 12-amp Hoover Elite soft-guard things and it works amazingly well in upright mode, but just decently well through the hose and still not as well as a clean-air machine. You have to manually switch it into attachment mode, there is not much suction as the house despite having more than an older Elite (that matters a lot to consumers), and the unit is remarkably noisy. Also, Hoover was heavily pushing their clean-air offerings at the time as the WindTunnel was their latest and greatest design. It made more sense to offer a basic stripped WindTunnel than two different models, though Hoover did not always do things that made sense in the 2000s. They certainly did not drop the direct-air models for no reason. I am sure that sales reflected that consumers preferred the WindTunnels.
Panasonic definitely got the clean-air design correct. Their Jet-Flo models were some of the best things on the market in my opinion. As other users noted, they had brush roll shutoffs, they were easy to change into canister mode, they were powerful in upright and canister mode which even meant that they were able to utilize extremely long extension hoses, they were able to make them very small and light depending on the target market, they were able to add unique features easily, and so forth. I always enjoyed how Hoover gave up on the design, but Panasonic took it and made it better and Hoover hopped back on the train.
I think the best place for a direct-air machine nowadays is for lightweight and/or cordless offerings. For example, Hoover/Oreck/TTI released their cordless version of the XL21 base and it is great. This makes sense because one thing that direct-air machines are is efficient. Even the corded Oreck models are nice in their own way. A lot of people like the lightweight build of them and I think that is why they work. Also, they are great for the elderly and disabled. Just have a little canister such as an Oreck Buster in the house and you are good to go. I know some folks do not like having two machines for the job, but there is a give and a take.
