Let me type my ruminations here. Just my 2¢ worth, but I think it is not a simple answer.
I believe it has more to do with the size of the gun buying markets in the U.S., vs. the markets in other countries. My guess here, is the U.S. sells more guns to American citizens and lawful residents, than all other countries, combined. Add to that, the popularity of the 9mm in the U.S. peaked within a couple of years of the major police departments here adopting the 9mm, like NYPD and LAPD, and then dwindled, nearly as fast, plus Americans tend towards "traditionalism". Great example is Harley-Davidson. The only major advancement they have made in engine design (the Revolution V-Rod), has a following, but not the huge following the traditional OHV 45° V-Twin has. I digress to make this analogy. This is similar to that of the 9mm, vs. the .45ACP. The 9mm has a dedicated following, but most of those who prefer the 9mm, also prefer the more modern design of the DA, or Safe Action weapons, kind of leaving the John Browning designs out in the cold, with fans of the 9mm. You don't see oodles and oodles of 1911's in 9mm being sold, either. 9mm users prefer the more modern design, and higher capacity of these newer high tech weapons. But, for the most part, Americans tend to like iconic traditional things.
Then there are those, like me, who respect new technology, but are not willing to compromise the safety of my family, on a lesser caliber, like the 9mm. So, I prefer big caliber guns, with modern designs, like the XD and Glock. But, being a fan of Jeff Cooper, I like major caliber weapons. .45ACP, 10mm, and .40S&W, are my top picks, but I am also beginning to become impressed with the performance results of the 9mm on steroids, also known as the .357Sig.
Maybe it's just a matter of time to accept new things, in the American market. But, I still contend that we sell more guns in the civilian American market, than all other countries' (civilian markets) combined. From there it is simply a matter of cartridge popularity. When my department decided to switch to the 9mm, from the .38 Spcl, I wanted to carry my Hi Power, but they would not accept any single action semi auto. Instead, I opted for the Glock 17, and came to appreciate the design, if not the caliber.