Why are there so many used B&W speakers for sale.

You have answered it yourself.

I can think if very few audiophile speaker brands that have enjoyed the enormity of success for as long as B&W has.  When you sell thag many for as long as they have you are biund to have a big presence in the used market as well.

+1 @erik_squires 

Mass market loudspeakers that people enjoy and many upgrade by moving up the line.  

+2 @erik_squires 

First excitement, then fatigue.

Initially they sound great but can wear on you over time, at least in my experience YMMV

"First excitement, then fatigue."

This supports my argument that we each need two systems:

A hot, exciting system that you turn on after coming home from work. A system that "gets you going" quickly for maybe an hour or two.

AND 

A more laidback system for the week-end that you listen to for hours-on-end.

+1 @petg60