I think there's 2 reasons for amount of used B&W's on the market:
1. They have been around for 60 years and have always had a pretty broad range of speaker choices, across many price points. They have also developed a wide range of retailers and distributors.
2. I also agree with some posts here that B&W's seem very forward and can be fatiguing, to my ears, especially at the lower end of their range.
When partnered with more laid back sounding electronics they can be a nice listen.