They are selling so many because over a year ago there was a special employee purchase price. They had to keep them for a year and now they are able to sell.
I don't know about Drken's explaination, but I sold mine simply because I didn't feel I was getting $12k of performance---the short version. I recall moments of magic when hearing the 803D's at my local dealer, magic I never duplicated at home. I tried various amps, including: Classe CA2200, Musical Fidelity kW500 integrated, McIntosh MA2275 integrated and McIntosh MC402.
There are others on here that to claim to be making wonderful music; I am not going to doubt it, they must have hit upon something I didn't and/or have a better room for them.
Keep in mind that B&W sells a lot of speakers, so the more you sell, the more chances of them being available on the used market.
I switched to Wilson WATT/Puppy 7's.