Once upon a time, the duty revolver in US big cities was the .32.  I don't think that changed until the crime waves of the '20s and '30s forced cops to go to bigger calibers because the now-mobile criminals (the relatively new-on-the-scene automobile was a boon to serious crime back then) were getting better armed; the cops were getting shot at by .38s, .45s and even the occasional Tommy gun!  Europe, with its generally more restrictive gun laws didn't see this surge in armed crime.  There the police sidearm was more a badge of authority than a defensive weapon - if worn at all! - so the .32 sufficed.  Even the .25 remained a "viable" choice in some cases!

At least that's how I sees it.

Stu