Why so many B & W Speakers on the used market?

I loved my b&w speakers purchase in 1991 and had for 12 years and gave to my brother when upgraded to 803s in 2004. Had those for 20 years before upgrading to 803d4 last year. First set did not sound as good in brothers system and he sold them. I used the speakers as the core and built the rest of the system around them and have always found them engaging, never fatiguing, and some of the most true to the natural instruments as I have found (and i have tested a lot of speakers in similar price ranges). But yes, depends heavily on the rest of the system. I think they pair poorly with mcintosh that they are often shown with. Due to listening to a wide range of music I have found some speakers that sound a little better with classical but lose the impact i need with punk. Others that have a little more edge but sound awful with acoustic instruments. So, results vary with your interests and taste. Also, with so many b&w options some will be a better fit for your system and interests than others... 802s sounded boomy to me and 804s anemic with the 803s wonderful...  But, all said I think they make outstanding products in almost every price range which is a true rarity.

I worked in listenup for several years, selling a ton of B&Ws as it was their primary line forever. Many many speaker lines are a little tipped up in upper freqs. Many many customers like that presentation. Others don’t.
 

None of that has anything to do with the number of speakers on the used market. As always, it’s best to go listen to anything you are interested in. 

...time for their annual migration....

At least the recent ones, often sound great in the showroom. But live with them for a month and they become screechy and unbearable. That's been my experience.

I'll echo what has already been said regarding the sheer number of B&W speakers out there in the wild. It has to be orders of magnitude more than almost any other high end brand.

Adding to that, they are sold by some of the more mass market chains like Best Buy via their Magnolia HiFi locations (which nowadays is usually just a section inside the store rather than a dedicated separate location). Last time I was in my local Best Buy the Magnolia section had Martin Logan, KEF, Definitive Tech, and B&W. The latter seemed to have the largest share of displays and marketing within that area. And as far as I remember they have always featured B&W, while some of those others feel like more recent additions.

It's easy for us forum users to think that Sonus Faber, Focal, B&W, Totem, Dynaudio, etc are all very well known and widely distributed. But I would guess that most "normal" people will know the B&W name before any of those others, by a wide margin.