As said, a lot of charity stores fell prey to virtuous dumping (or plain old dumping). It's cheaper that paying the council to dispose of stuff. They can't store it and often can't sell it, so end up having to foot the costs.
It's true that some people have odd perceptions – we have a pile of Ikea cupboards and bookshelves in the garage, which aren't in perfect condition, but they're not in bad condition either. Couldn't give them away (some quite odd people came around, I mean really, it wouldn't be free if it were brand new and in perfect nick, would it?) Someone actually asked if we could deliver it.
On the other hand, when we had the kitchen ripped out, no sooner than it was sat on the drive, someone pulled up and asked us if we still wanted it, and despite its condition it was gone in a day.