I think people that buy them tend to hang on to them and don't sell them.
That may be true, but a more likely explanation is that CZs are just now starting to be known to the wider shooting population. (CZUB and CZ-USA are small companies that have had to start from scratch since the fall of the Communist Bloc, and they didn't have the big bucks to spend on marketing and advertizing.)
Just a few years ago, on these gun forums, darned few people even knew what a CZ was. There was, back then, no awareness of the brand, no advertizing, no shooting teams, etc. Things have changed.
That said, there are still a LOT FEWER CZs in the hands of shooters than Glocks, S&Ws, Kahrs, etc. Ten years from now I think you'll see a different story when discussing the availability of "used" CZs.
I was a senior moderator on the original CZ Forum for a bunch of years, and the thing I noticed then was that people new to handguns seemed to gravitate to CZs, while the experienced shooters (SIG owners, S&W 3rd Generation, 1911s, etc.) tended to shy away from them -- they didn't see any value in them. A lot of rifle enthusiats got a CZ as their first semi-auto pistol. That attitude and purchasing behavior has dramatically changed in the years since I left my role as moderator on the forum.