It is the cost of ammo.
Over the last 2 years there's been a trend back toward 9mm which is mostly driven by the economy. Many shooters who were shooting mostly 40, 45 or 357 Sig tended to fall back to the 9mm due to the lower cost of plinking and practice ammo.
Some shooters simply said it was due to the cost. Others justified it based on the performance of upgraded jhp over the last decade or so ("the 9mm will do just as well if you do your part") etc. Some discovered that the recoil of the 40 was suddenly not worth it and that they shot the 9 better. Others that they carried more rounds of 9 than 40 in a small CCW.
There are a lot of particular reasons but it coincides with less disposable cash and the gradual increase in ammo prices.
I've talked some with gun store owners about this for the last year or so. I noticed more Fo-tays showing up on the used gun shelves myself and everyone has noted the increasing cost of ammo.