The FBI released a years long comprehensive study of the effectiveness of various calibers of semi-auto handguns and their use in the field. The result was the FBI going back to the 9mm from the 40. I imagine this study has a lot to do with it.
The .40 was really hot during the mid to late 90s. The bloom seems to be off now.
I have noticed when CDNN lists pistols for sale, .40 caliber seems to be $10-20 less than the same gun in 9mm. It is actually a perfectly good cartridge and I might would take advantage of the savings if I had the money to buy one now.