Oddly I was wondering the same thing, and I just started selling Rainbows with my wife as a team.
A brief history here first, I've been a believer in bagless vacuums since the early days of Fantom and have more vacuums than any normal person (I've got 3 Kirbys and a Fantom Cyclone XT in my garage at the moment).
Shortly after we bought our house back in 2009 we ended up getting a Rainbow D4 via inheritence, complete with everything except the powerhead.
About a year later I scored an E Series in like new condition at a thrift store for $19.96. I literally ran to it and grabbed it when I saw it. Even around the local Rainbow office everyone is envious of that find.
We actually approached the Rainbow dealer a couple of weeks ago to see about getting fragrence oil since we didn't know where the local distributors were. The guy offered to demo us the new one and take a look at ours in case it was in need of service.
When he got here he saw the D4 and E Series and he took a look at them which were obviously in impeccable shape since I take care of this stuff. Gave us a 15 minute or so rundown of the new model and asked us if we wanted to sell them as a team. My wife loves the thing and despite both us having full time jobs here we are selling them. We don't really need the money I think its the fact we're the exact demographic they're looking for to sell to young married homeowners.
It does what it claims to do and costs a lot. I guess it depends on your logic as a buyer, Rainbow stands behind it well and it holds its value. Most average people never set foot in a vacuum store unless they need a part. Most go to Wal-Mart or Kmart and plop down their $75 every year or two for a new machine.
Rainbow's sales tactics are both good and bad because a lot depends on the local distributor. A good distributor is honest and upfront showing the machine's merits, the others are sketchy and pushy. We have two locally one has a 5 star rating and the other a 2 star.
Like Kirby the thing here is the vast majority of the population is not going to walk into a store to purchase an expensive vacuum. Heck even Dysons are a tough sell. You really have to get the person to realize that that $49 Bissell is going to be a yearly investment unless your house is tiny and you only vacuum once a week.
I see the logic behind retaining a door to door type business. If you put it on the shelf at Kmart I guarantee you no one would touch it. It is too expensive to sell in that fashion, I'm sure its not an easy sell at a higher end vacuum shop with used models.
I don't even do most of the vacuuming, I'll ocassionally do so if my wife is sick or something. I'm not the biggest guy maybe 5"11 just shy of 200lbs but that Kirby is a heavy as heck, I only use them to vacuum the garage carpet. At least the Rainbow is manageable.
Usually we alternate between the Dyson DC07 upstairs and the Fantom Thunder, Dyson DC33 downstairs and the Rainbow doing both every other time. The biggest sole advantage is you don't get a massive dust cloud emptying in the bin. Like any of the other machines it doesn't lose suction, and anyone claiming it doesn't filter well is more than welcome to come here and see the nice white never changed filter on my E Series.
I think Rainbow could transition the design to an upright which might appeal to a larger demographic. Making a durable upright would probably be fairly heavy.
The other thing they could do is redesign the water pan, I'm sure there are many ways this could be done, to make it easier to remove, fill and drain.
The Rainbow is one of those things you either like or don't. Despite its high cost I don't consider it a bad deal since they tend to hold up well and retain their value well.