I have a theory, someone is bound to disagree and there are exceptions, but here it is. I think Warwicks are much more appealing to people who haven't figured out the exact details they like in a bass yet, partially because of Warwick's incessant marketing campaigns and flashy woods (which, I think, they spend way too much time on and their product suffers from it, rather than researching ergonomics of the average player, for example). I think a lot of people lust over them, particularly newer players looking for their first 'upgrade bass', and then later realize owning one isn't all its cracked up to be for whatever reason (again ergonomics, which many many players do not mesh with).
Now, when people love Warwicks, they absolutely won't play anything else. But it appears as though there are a lot of people who think they are exactly what they want, having not owned one yet, buying it and inevitably figuring it out they can be a pain in the... um, shoulder... and back, and wrist.
I can speak on this because I went through it myself, I was hypnotized with the thought of the Thumb, got one, and then a few months later realized my playing suffered as a result of it. When I went to sell it, I think a lot of people who are already buying basses on TB already figured this out. As a result, there are a lot of people going through the same cycle, and the pool of people who are willing to buy another Warwick dwindles. Supply and demand rears its head.