Why Couldn't They Use a .45?

I thought the reason the army used to use a .45 colt years ago was because it could knock a man down. The Beretta9 apparently couldn't do as much:
http://www.cnn.com/2017/01/19/politics/army-new-pistol-sig-sauer/index.html

Rod Kaufman said:

I thought the reason the army used to use a .45 colt years ago was because it could knock a man down. The Beretta9 apparently couldn't do as much:
http://www.cnn.com/2017/01/19/politics/army-new-pistol-sig-sauer/index.html

The Sig is modular and could use the .40

Did you ever shoot a 45ACP? A link to compare.

http://www.ballistics101.com/45_acp.php