Everyone is getting a 9 lately. It's the 'in' thing to do right now. Cheap practice gun, and a compact carry piece that is batter, ballistically, than the .380 (aka 9mm short) and the .38 Special.
Why is it better than the .38 Special, though? The case is smaller looking side by side. 9mm is better than .38+P even. The projectile is essentially the same, the bottle the .38 has to store propellent is larger, how did this Luger Parabellum fella get more oomph in his cartridge 110 years ago?
The case on a .38 is a whole centimeter longer! And it has, generally, half the foot pounds of force compared to the 9mm.
I suspect the .38 standard was because the parent casing was developed off of black powder catridges. While it COULD be stuff with more powder that would be bad for guns not designed for that force. So the standard stays what it was originally. Just like the 9mm stays the same. The .38 Special was only developed 3 years earlier than the 9mm parabellum.
And .38 WAS ramped up to the .357 Magnum. MUCH more oomph than .38 or 9mm. They made a longer case for the .357 to keep the round from fitting in a .38 pistol. That whole gun-explodey thing was the worry, I understand. But perceived recoil is much more severe in the .357.
What I'm saying is, why wasn't the .38+P done up to match the teeny weeny 9mm? Does the fact that the 9mm is for a semi-automatic help? That 9mm was specced to accept a LOT of pressure, too. More than any of the .38/9mm projectile cartridges save the .357 Sig.