As far as I can tell people started shooting 9mm a lot (again, after its decline in the 1980s after everyone decided it was garbage) in the early 00s when the hard core training community exploded. Why wouldn't they, it is a hell of a lot cheaper than .40, 45 or 10mm. Most of the trainers , when it started, carried .45s. Oh, and everyone said that SMG were garbage and everyone wanted SBRs. But after a while all the trainers started promoting 9mm, because it was more conducive to high round count gamer style training--especially after the expiration of the mag ban. By the way, the despised James Yeager was WAAAAY ahead of the curve. Trainers live in a James Yeager world now (Glock 19, 9mm, 1000 round mag dump class...ok but even he couldn't trick everyone into carrying those awful big dots).
There was a period , not coincidently during the hottest part of the wars, where the military was trying to dump the 9mm and go with .40 or .45. But the wars wound down and they just decided that a cheap plastic 9mm would be fine, since pistols are basically nonsense. Combine this with the Obama-era FBI moving back to 9mm because they found it was easier for the new crop of more diverse agents to qualify with. Now I even see people pushing these PCC, which are basically just back to SMGs with no fun switch. It is fun watching the wheel turn.
I'm mostly a J-frame guy, so I am just an observer here! OK I'm trolling but there is some truth to what I say!