Me thinks its ammo cost.
I disagree. I think that the vast majority of gun purchasers don't really factor ammo costs into their buying decision - they just go on the assumption that ammo costs are relatively about the same from caliber to caliber. The 9mm is a very advantageous caliber for concealed carry (low recoil, less weight, high capacity, sufficient stopping power, etc...). Yes, it is a trade off,
but everything about concealed carry is a trade off.
45 ACP is without a doubt the ideal pistol cartridge for self defense, but it does not always lend itself to ideal concealment options, and especially not deep concealment. If your particular concealed carry options dont allow you to carry a mid to full sized .45 everywhere you go, then I'll take a 10+1 subcompact 9mm over a .380, .32, or a .38 special wheel gun any day of the week! -------Make mine a Glock 26!