9mm is the most commonly sold centerfire pistol round in the U.S. Before that was true, I suspect that the .38spl held the honor.
For the bulk of the 20th century the .38 SPL was the king of centerfire handgun cartridges, by sales, in the US. Also led the pack in reloading die sales as well. It wasn't until the last decade of the 20th century that the 9mm passed the 38 SPL in sales.
Want an interesting trip through history (if you aren't old enough to have loved it)? Find some old gun magazines from the 1950s and up. (Older ones are much more rare), and just look through them. Look at the articles and the ads. Truly an eye opener, both about the changes in firearms technology, and attitudes.
Today, I gave a friend a present, a nearly mint copy of Guns & Ammo from the month and year he was born (10/59). Today, you can't pick up a gun mag that covers handguns without at least one mention of the 9mm (not counting the ads). It didn't use to be that way. I have a collection of a large percentage of the G&A, Shooting Times, and American Rifleman covering the last half century (not to mention living through it), along with lots of other magazines like Handgunner, GUn World, etc. Things have really, really changed in the last 25 years, and if you don't know it, you ought to take the time to find out.
9mm Luger (and I still call it that, a rose by any other name....) is king today, and very likely going to stay that way until we pass beyond cartridge firearms for peace keeping use. Because it does work well enough (and better today than in the past for the best loads), and there is a huge fiscal investment in it, worldwide.
The basic assumption underlying some of the comments here is that American pistoleros used big bore handguns. They did but more used something else. The .31 caliber Colt model 1849 was supposedly produced in numbers of around 335,000 during the time it was in production, which was only 23 years. The Colt Single Action Army, with variations, on the other hand, was produced from 1873 to 1940 before production was suspended until the mid-1950s, and only about 357,000 being manufactured. About half of them were in .45 Colt. Around ten percent were in calibers smaller than .38-40.
As to comparing the numbers made/sold between smaller calibers and bigger ones, there are lots of other factors involved than just bore size, in popularity. Americans are big bore enthusiasts, the numbers of cartridges and guns made and sold in big bore calibers prooves this, beyond doubt. What we aren't is big bore enthusiasts to the exclusion of all else.
I know some folks who are big bore guys, won't consider anything under .40 as useful (other than the .357, and sometimes not even that), but you know what?, nearly every one of them has a .22LR. Often, more than one.
The most dedicated big bore fan might also own a .32, .380, .38 or 9mm as a back up or CCW gun. And lots of folks who aren't big bore enthusiasts might own a .45 or a .44 in their collection along with many smaller guns.
I'm not a 9mm guy for my personal defense. I like the big bores (.45 mostly) for that, although lately I have learned that there is also value and use to smaller rounds in pocket guns. And I own (currently) 5 pistols in 9mm Luger. 3 of them are historical pieces. A P.08, a P.38, and a Broomhandle Mauser. And a T/C barrel in 9mm. After years of sneering at the 9mm (and not without reason), I have finally seen the light. I no longer sneer.
I'm not an advocate, for my uses there are better choices, but not always a lot better...
Today, 9mm ammo is not what it was 40 years ago (US made) and foreign ammo is common. All those years of 9mm vs .45 articles still have some valid points, but not as many as they used to have.