Why? Because the 230 gn ball load from the 45 acp was designed to replace the 45 Colt round, which was designed to de-horse a rider or falter his mount at 50 yds. Please keep in mind that when the 1911 pistol was first issued to the Army they still had mounted units, not like obama's believe about bayonets.
If a 230 gn ball can knock a man off his horse it is good enough for me, and it was. On a personal note, it worked just fine for me in both southeast Asia and Latin and South America. One may feel the need for some zombie max type ammo when dealing with drug zombies and the like but I have never seen two rounds of 45 ball, or for that matter, 3 rounds of 9mm ball fail to stop a fully armed enemy soldier in full battle dress intent on putting an end to you (me, actually). That's why. It works.