Trying to think "why" for the 9mm revolver.
Only thing I can come up with is that some PD someplace wanted revolvers for the officers that, for whatever reason, could not handle the 9mm automatics that were issued. That could be officers lacking strength to rack the slide, or recharge a magazine, manipulate a safety, whatever....
Or, it could have been a training thing, whereby, officers already very familiar and good with revolvers could opt to the 9mm revolver. I can imagine certain, perhaps longtime, detectives and such, wanting to do that.
Could also be public awareness - where they just wanted certain officers to be equipped with revolvers "just becasue" the public expected the officers to be so equipped.
Keeping in mind that policing is as much, or more, a political job as opposed to a crime fighting job.... ya know... ya never can tell