Because the 0.45 ACP is what they are used to. It has a lot more to do with tradition than informed opinion. SF operators like .45 ACP because it's low muzzle velocity and heavy bullet make the round suitable for used in silenced and suppressed weapons.
Just for the sake of curiosity here are some specs for popular pistol catridges.
9x19mm UK Mk 2Z Ball: muzzle velocity 396 m/s, muzzle energy 584 J
0.357 Magnum: muzzle velocity 436 m/s, muzzle energy 972 J
0.40 S&W Ball: muzzle velocity 300 m/s, muzzle energy 469 J
10x25mm Norma Ball: muzzle velocity 340 m/s, muzzle energy 618 J
0.45 ACP US Ball M1911: muzzle velocity 250 m/s, muzzle energy 474 J
5.7x28 FN Ball: muzzle velocity 715 m/s, muzzle energy 516 J
5.56x45 M855 Ball: muzzle velocity 930m/s, muzzle energy 1,708 J
7.62x39 PS Ball: muzzle velocity 725 m/s, muzzle energy 1,993 J