Because modern science has proven that the .45 is based on old world ammunition science. Back in the .45 ACP days a bigger bullet meant a better round for killing. The US military proved that the .45 ACP was superior than the 9mm. However with the advent of JHP and modern ammunition manufacturing a round must achieve a certain level of penetration and expansion to be considered a lethal round. A .45 JHP versus a 9mm JHP have almost identical expansion. A .45 can expand around 5-6 inches versus a 4.5-5 inch expansion out of a 9mm.
A common misconception by civilians is that special forces/ navy seals / marine recon / etc etc use .45 ACP. The only reason why those units use the .45 is because of international treaties barring the use of hollow point ammo. Without expansion a .45 ACP is a drastically much superior round. Wound channel ballistics of a .45ACP versus a 9mm WITHOUT expansion can be over 35 percent more lethal. HOWEVER with expansion the difference is less than 10 percent in most cases.
This is why the FBI is going back to 9mm and this is why the .45 is entering the same realm as many guns we thought to be the epitomes of manhood.
Or simply. Because technology
Oh! No,No,No! Now you've done it! Equated manliness to bullet diameter!
It is perfectly obvious that manly men carry .45's and girly men carry 9's!
We 'don need no stinking scientific evidence!