The way I understand what I've read on the subject & a thought on the 9.
First of all, the 9. After WWI & for many years, lots of folks turned their nose up at the 9, as it was connected to the dirty low down Germans. Sure, there were bring backs in a lot of European, or German cartridges, but American ammo manufacturers avoided making many calibers. Even in the 60's, Norma 9mm was about the only factory chambering seen for new factory ammo. It just hadn't been accepted or taken serious, yet.
Now, the 38 super. Read that law enforcement officers/agencies of the 1920's requested something to help penetrate the car bodies of the day and helping fight the gangsters of the era. Note, Wiki indicates this as well. More powerful than an unpopular German 9 & the Super was American. Win/win.
The 357 magnum came along not too many years later & armor piercing ammo was popular for a time.
