Why are you puzzled there is no shortage of used planars?
There is no shortage of used box speakers either. In fact, there are far more used box speakers (because more are sold).
By my quick calculation, just looking at audiogon, there are roughly 480 speakers for sale, with only 24 of those being planar speakers. 5% of the speakers for sale.
Does this constitute some surprising percentage of planars for sale?It doesn’t to me. Especially if you take in to account that this is a high end audio site and planars are virtually automatically "high end" speakers. (In other words, there is a selection bias going on - there is a much higher percentage of box speaker to planar speakers sold if you take in all the "non-high end" box speakers sold. But once you are talking high end audio, it’s almost a right of passage for many to have tried planars at least once).
Anyway, if you are looking for reasons some of us move on from planars, I moved on (from Quad ESL 63s) because although I loved the boxless quality and sense of transparency, I missed the sense of body, palpability and dynamics I get with box speakers. And these days, plenty of box speakers sound very "boxless" and transparent, so I’m getting enough of that plus the dynamic qualities I crave. I’ve seen many previous planar owners express the same sentiment.
Of course, there are others who had a planar speaker early, tried box speakers, found that box speakers didn’t give them what they liked from the planar, and went back to planar. And so it goes....