This is because the bullet of 9mm size produces a superior entry would to larger caliber weapons and is unlikely to pass through the target so much as bound around inside and chew up internal organs.
In order to make a superior entry wound a 9mm has to enter the target. With modern body armor in todays urban warfare and the no doubt increasing effectiveness of said armor. Even today pistol/smg rounds and somtimes even the 5.56 have trouble punching through this armor.