Mike, probably because recoil is less thereby allowing for faster and more accurate follow up shots. Also some folks just can't handle a 45. Heck, even a lot of police departments are allowing or switching from 40 to 9 because of some female and smaller statured males just can't handle the recoil of a 40. AND with all the advances in ammo that is going on the difference between 9mm, 40S&W and 45acp are getting less telling all the time.