Multiple reasons, none of them valid, IMHO.
#1 Claims were made about difficult logistics with NATO "allies" who used the 9mm. Somebody figured it would speed resupply to have to carry just one handgun caliber. Musta been a frog.
#2 Claims were made that the 1911 was too much of a "kicker" for female soldiers. Well, sure. But I taught my daughter to shoot when she was 10. Today at 24 and married she has her very own Ruger SP101 357 magnum. She's a bullseye shooter with that gun at ten yards with a 3 inch barrel. Recoil can be overcome by proper instruction and practice.
#3 International Politics coupled with...
#4 Lots of money.
Its a damn shame that US soldiers carry a weapon not even made in the USA. Its a Beretta if I'm not mistaken.
If (a big if) I were POTUS I'd order every Army and Marine ground pounder to carry a .45 ACP that was Made in the USA with ammo Made in the USA. Mandatory range practice at least twice a week, shooting at least 5 clips on the 7 yard range.
The Euro's can jump in a lake. We don't want to follow what the Euro's do, never. We're Americans and we can do anything better than a European can do.
you also forgot capacity. same reason why they went with 5.56 over 7.62. you can carry more smaller bullets at the same weight.