It is definately due to the cost difference between 9mm & 40 or 45. For some reason 9mm has come back down in cost but 40 & 45 remains high.
For me I see a lot of first time gun owners (many I know) buy .40 caliber. It is kind of a catch all caliber. As already stated, it fills the role of capacity and power combined.
After an owner gets comfortable I think they tend to move toward 9mm (price, capacity etc.) or to .45 (caliber, one shot one kill, stopping power, cracking engine blocks etc. etc.)
9mm for me (I admit I do like .45 too)
Everybody is drinking the 9mm Kool-Aid .
There is no such thing as one shot one kill. The whole myth of the .45 being the supreme man stopper is just that, a myth. If you look at solid balistics testing the 147 grain 9mm, 180 grain .40, and 230 grain .45. Have basicly the same depth of penetration. Causing only a slightly larger wound cavity as you step up in caliber. Probably 90% of LE agencies, and including Homeland Security use the .40. If you want a one shot man stopper, then you might want to consider carrying a shotgun or high powerd rifle. This is not hollywood where pistol mags hold 50 rounds, and people get blown back 5 feet from a single handgun round. Enough said!