The .45ACP has been around for 100 years. It has been the standard US military pistol cartridge for a good portion of that time. That's a pretty good head start over the 10mm.
The 10mm is a good cartridge. I like it, but it has too much recoil for most people, it creates a larger pistol grip than the 9mm and .40, and it is expensive and hard to get a hold of.
The fact that the .45ACP has been around so long, is so much more available and common, and fairly similar in performance (maybe slightly less effective) leaves very little room for the 10mm to get in on the action.
How many agencies issue 10mm as their standard caliber? Can't imagine there are many...