Well, just how many police agencies in the United States use a handgun in .45 ACP for regular issue? This may have been covered in the thread about why they didn't use the .45 Colt in revolvers.
At this point in time, however, it is difficult to believe that the 9mm was not always so common, nor the .45 so old. The .45 auto was used just about as much as the 9mm before WWII. If I am remembering everything, the 9mm was standard only in Germany, Poland, Belgium and probably Austria. It was also standard in one or two other places. Italy, Hungary and Yugoslavia used the .380 while France was still getting by on .32 automatics. Finland used the .30 Luger. But the .45 was used in Norway, Mexico and some other odd countries. Even Great Britain used large numbers of .45 autos, though not all were in .45 ACP.
While I use the word "standard," it does not follow that any given standard weapon was used exclusively and following WWII, some countries like France (again) used whatever they could find.
The German police forces, always the most interesting, attempted to upgrade to something more powerful than .32 but still not a 9mm by adopting something called the 9mm Police, if I'm remembering the correct designation. It was about like the 9mm Makarov but not interchangeable. Pistols were made by Walther and Sig and they are something rare today.
I might also note here that in trying to find out a little more information about those 9mm Police autos (which I didn't find where I expected to find it), I did run across many references to Lugers, mostly, as used by Germany police as far back as the early 1930s as well as their use post war as well in both East and West Germany. So while German police did in fact use .32 automatics, their use of 9mm pistols goes back a long ways.