IIRC, 9mm is also a standard Police issue caliber.
It was, but departments are trending toward the .40S&W of late (started with the FBI, IIRC - although HRT still sticks with .45ACP - again, IIRC.)
"What is Popular is not always Right" - the US Armed Forces uses the 9x19 because the rest of NATO uses it, and they refused to move up to something more useful (although the 9x19 is a bit more powerful than the 9x18 Makarov - more or less standard ComBloc for a while.)
But, if the 9m/m is so damned efficient, why are there so many iterations? Let's see - I probably don't even remember them all, but here are the 9m/m auto calibres I do recall:
9x17 (.380ACP. Designed for work against dogs and small threats, IIRC.)
9x18 (9m/m Makarov)
9x19 (9m/m Luger or 9m/m Parabellum.)
9x21
9x23
9x25
The latter three started out as wildcat loadings to "make Major" in IPSC competition - because, apparently, either people didn't feel up to handling the .45ACP (which isn't that bad to shoot - I can dump a full mag downrange with reasonable accuracy, and the pistol looks like it's vibrating in my hand...) and there are more "tuner" parts available for the 1911
et al (1911A1, 1991A1, Commander, Officer, SVI 2001, ...) than pretty much any other pistol out there. The 1911 is like a Harley - you can, with a little bit of looking, build one
entirely out of aftermarket parts! And have it be at least as good as most factory offerings (it takes a fair bit of work to equal a high-end Kimber or a Les Baer, but it's doable.)
Yes, if you want to get pedantic, you can include the .38 Special, .357 Magnum, and .357 Maximum as being "in the 9m/m persuasion" - but I'd consider all of those superior to pretty much any 9m/m auto loading - both in terminal ballistics, and in general platform quality.
The various PDs probably went to the 9m/m for the "Accuracy by Volume" tendency as I mentioned before - but if you're not going to hit your target with two rounds, throwing 17 at it won't do much better. cf: Amadou Diallo - 41 shots fired, 19 hits, three cops, contact range. If someone working for me shot that abysmally, I'd relieve them of their sidearm until I was happy with their retraining. Go to hand-to-hand for a while...
(Yes, I know about the effects of physical/psychological stress on frangible physical skills. That's why stress should be induced in training - if you want to find out how good you are, try this: Lay your sidearm down on the firing point (select one for fifteen yards.) Do twenty press-ups,
run flat-out 100 yards out and back, and do another ten press-ups. Then, pick up your sidearm and shoot for record.
(Repeat until you're happy with the results. Don't fire more than, say, three double-taps at a time. You won't get "competition accurate" doing this, but you can make it to "combat accurate" and your confidence will skyrocket.
(Yep, I'm a pure bas***d as a trainer. The confidence of my trainees is terribly high. I see no reason to discontinue the practise.)
This is why terminal ballistic performance is so important in a defensive sidearm - if it comes down to SHTF, you're not going to be shooting like you're point-shooting at a range. You're going to be under a phenomenal amount of stress (the idea behind the running and the press-ups is to simulate the effect of adrenaline upon your system and co-ordination.) If you can put two rounds steel on target, land them all of about a couple of inches apart, and do that
every damned time, you're going to be shooting better than probably 70-75% of the boys in blue out there. Most of them practise point-shooting, but stress isn't simulated in their training except for maybe the "Hogan's Alley" exercise in most academies.
Sorry - Hogan's Alley should be done a minimum of once per week,
after a half-mile
run, and done with any duty weapon available to you - sidearm, patrol carbine, rifle, shotty, whatever. If you might carry a SMG on duty, include that in your rotation.
Does this sound excessive? Most people would think so. But, if your life (or the lives of others) is going to depend on your skill with firearms, you'd better take
every opportunity to improve that skill you can find!