Most states allow concealed carry so the emphasis is on small, concealable guns. The 9mm excels in this area. So the general consensus is that the 9mm with modern ammunition is fine for most applications where a automatic handgun makes sense. If you want more power than the 9mm, the .40 doesn't make much sense, because it is such a marginal step. Arguably the .45 is just the same. Not everyone is enamored by the .45 these days either. Neither the .40 nor the .45 offer enough of an increase in performance over the 9mm for me to consider them worth the decrease in capacity and the increase in weapon size and ammunition cost. So the .40 is taking a hit in popularity because a lot of people are asking what some of us have been asking since its inception, which is "meh, why bother?" Meanwhile the 10mm is enjoying a, increase in popularity because it represents an appreciable increase in performance over the traditional auto pistol cartridges. If you want real performance from your automatic handgun, you get a 10mm Auto, otherwise, The Nine is fine.