They want low recoil for women troops and/or non-experienced males, penetration of body armor (although maybe not the solid plate armor which even a rifle won't penetrate), high capacity magazines, etc. Lightweight ammo wouldn't hurt either.
I'll make a broad statement and say that for the USA, the 20th Century belonged to the .45 ACP - an auto pistol version of the "big, slow and heavy" .45 Colt. Perhaps in the 21st Century it might be time to look at the 5.7x28.
I don't know a whole lot about the round other than its metric designation but just throwing it out there to get out of the 9mm-40-45 box.
Gunnery, gunnery, gunnery. Hit the target, all else is twaddle!
