For on-person carry, the key is proper penetration without over-penetration and comonality of round. The first one is obvious but let me explain the second:
If, god forbid, you have to commit to drawing and firing your weapon, as a gun owner you're now on the spot. Why did you have a gun? Why did you shoot him? Where did you shoot him? Did you shoot first? A myriad of questions come at you , and you have to be prepared for anything. If you shoot someone with a .45, but the bg only had a knife, this could make your life a living hell far beyond the incident. It's the same with those who carry Hydrashock rounds in their carry gun. If you do have to fire it, the police are going to ask "Why do you have rounds that are MEANT for human targets and are meant to cause the maximum amount of damage" especially if the BG has a far lower powered weapon. Smaller weapons or less advanced weapons equate to "less threat" in the legal world.
At the same time, you don't want to be caught with a .25 when the OTHER guy has a .50. Then you have the opposite problem!!!
The 9mm is a popular US round because it hits the target, and rarely goes beyond that. It's an easy round to control, place and fire without risking it going beyond the target. This is also why the .32 is the most popular civilian round in the EU, because penetration is a HUGE concern in the more densely populated countries where the right to bear arms is also legal.
It's a fine balance that I think the 9mm fits just right. Not too much, but just enough for what the common man faces on the street.