Easily explained by two factors:
1) The 1994 Clintoon Assault Weapon Ban that also limited handgun magazines to 10 rounds or less had an interesting byproduct: The sudden rediscovery and renaissance of the .45 caliber 1911. If civilians were to be limited to 10 rounds or less, they looked towards larger calibers.
2) During this time period the FBI, followed by local and state police forces, then subsequently by civilians, switched from the .357 and 9mm to the .40 S&W. That trend is now reversing, with the FBI and police departments returning now to the 9mm. This is providing the market with a glut of police trade-in .40 S&W handguns that are not moving very well.
I think that was a mistake. With speed loaders you can reload pretty quickly, and 10 round magazines just encourage "spray and pray".
“The paper found that since 1994, the manufacture of .38-caliber guns rose 12-fold, with nearly 820,000 of the pistols produced in 2015. The manufacture of 9 mm guns also increased seven-fold in this 15-year stretch, while those larger than 9 mm jumped 150 percent
“Easily explained by two factors:”
You fell for it. They changed the measurement system to make it look like ‘larger than 9mm’ had a HUGE jump....
Translating what they said shows this:
.38 guns went up 12 times
9 mm went up 7 times
bigger than 9mm went up 1.5 times.
So the larger calibers are getting more uncommon (by proportion).