Heh. It gained popularity because the FBI was using it. They started to use it because they liked 10mm after the Miami shoot-out fiasco, but 10mm was too much for agents. So they issued the 10mm-lite(.40S&W). So a bunch of LEAs followed suit. And everyone thought it must be more effective than 9mm because it has more kick. And the .45 ACP crowd was happy because it started with a 4.
Then, the FBI decided to go back to 9mm because agents were not qualifying well with 40S&W. Their accuracy and follow up shots were better with 9mm, and their ballistics testing lab determined that 9mm with modern JHP was just as lethal as 40S&W, especially because shot placement was easier with 9mm. Then, everybody followed the FBI again.