Many reasons
I carry and shoot both the .32ACP and .380ACP.
I started shooting the .32ACP on a regular basis during the first OBAMA ammo scare when I found that I could buy .32ACP ball ammo at normal prices, but .380ACP was scarce and usually sold at scalper prices.
Since then, I have acquired several .32ACP pistols. I carried a WALTHER PPK and also bought a WALTHER PP, both in .32ACP. The PPK was the hands down choice between these two. The guns look similar, but the PPK has a much more ergonomic grip shape. I could shoot over a 120 rounds at a time in the PPK and was not fatigued by it. With the PP, 90 rounds was painful. The difference was in the rear of the grip. The PP is squared off and the PPK is rounded, this made a huge difference to the web area of my hand.
I used a FEDERAL Hydra Shok in the chamber and loaded the magazine with COR BON Powerball. The Powerball feed 100 % and will always expand.
When the ammo scare ended, I acquired a SIG 232 in .380ACP and found it superior in every way to the WALTHER PPK, except for bulk. They weigh about the same, but the SIG is closer to the PP in size.
The SIG has night sights, much better trigger, a very comfortable grip for me and a hammer dropping lever instead of a safety, which unlike the WALTHER, you cannot forget to return to "OFF SAFE".
Because of the ergonomic grip on the SIG 232, the .380ACP recoil feels the same as the .32ACP WALTHER PPK.
If I was looking for a holster gun or pocket gun, I would go with the .380ACP.
You have a wider choice of ammo, some of it more powerful or effective than the .32ACP. The fmj practice ammo is cheaper in .380ACP and most important, you have a much wider choice of new production guns like the SIG 232, RUGER LCP and GLOCK 42.
I replaced my WALTHER PPK with the GLOCK 42 because I found it just as comfortable to carry, as easy to conceal and I found the perceived recoil of the .380ACP fired in the GLOCK 42 was about the same as the .32ACP in the WALTHER PPK.
I would feel safe with either gun, but the GLOCK offers more power and a simpler method of operation.
The pluses for the .32ACP are less recoil in the same size gun and equal recoil in a smaller gun, but not always. I found that guns with polymer frames shoot softer than metal frames, so the very light KEL TEC 32 is more comfortable to shoot than the all metal (and heavier) NAA Guardian.
I also find, that guns originally designed to shoot the .32ACP may not work as well in .380ACP, but not always.
I sold my .32ACP WALTHER PP because it was not a comfortable to shoot as the smaller PPK when I found a much more comfortable to shoot replacement. The BERETTA model 82 Cheetah pistol in .32ACP. This gun reduces the .32ACP to nearly the level of a .22 rimfire for me and has proven 100 % reliable with any ammo except the WINCHESTER White Box fmj's which are unreliable in every gun I have used them in.
I also had a COLT 1903 Pocket pistol which was very reliable and accurate, even using hollow point ammo and was a joy too shoot. It was a great range gun, but lacking a double action trigger or safe action trigger (like the GLOCK), I considered it unsafe for self defense.
If the recoil of the .380ACP is excessive for you in the size or model of gun you want to use, then a .32ACP could make a lot of sense.
It is better to have a .32ACP that you can shoot well than a .380ACP that you struggle with.
Jim