My guess. People are taking advantage of the premium price used hi-cap guns are bringing to clean out the safe of guns that just don't get shot as often. And .40s cost more to shoot than a 9mm.
I sold my glock 22 a while back because it routinely got left at home in favor of other similar models, and the used market is very profitable. If you ever thought you were going to unload a .40 gathering dust, now is the time.