Top 10
1. For any given capacity you get a smaller, thinner and lighter handgun; or in equal size gun you get more ammo and still lighter gun.
2. Less recoil = higher probability of follow up shots under stress (likely one handed grip with hand tremor). Multiple shots on target is far more important than slightly larger hole.
3. With proper ammo just as killy as .40, .45, etc.
4. It has killed more people than all other handgun calibers combined.
5. Countless armies and police units across the globe prefer it.
6. Universally available.
7. Better priced than other respected SD rounds. Because it’s not about the few dozen rounds you carry on you, but thousands you put downrange.
8. Greater variety and availability of gun choices chambered in 9mm than .40 .357, .45, etc.
9. Given same rd capacity, spare mags are lighter and smaller.
10. Because .380 is not killy enough.
The real question is not why someone would carry 9mm, but rather – why would anyone carry anything else.