The adoption of the .38 Colt was pretty much in line with the general military handgun trend at the time, away from large (10-12 mm) caliber and to small (7-9mm). The same trend took place in rifles as well, and the U.S. was not significantly behind other nations in either respect. The reason was that lethality could be achieved either with a big, slow bullet, or a small, fast bullet. As powders improved, the small fast bullet became practical, and that was the trend. (The same argument still persists - the only difference is that the rifle calibers have changed from .45 and .30 to .30 and .22 or .17.)
The only nation retaining its .455 caliber revolver was Britain, which contended that the big bore was needed to deal with folks who weren't "civilized" enough to know when they were shot and just fall down. Most European countries kept the small caliber, but the U.S. went back to .45 when it encountered some "uncivilized" people.
Ironically, when the British did decide to do away with the .455 revolver, they went to a .38 caliber that was not any better than the .38 Long Colt that the U.S. had rejected 30 years earlier.
Jim