The military decided it had had enough of the safety issues inherent in a single action pistol. You cannot chamber a round and lower the hammer without the risk of an accidental discharge through hammer slippage. You cannot chamber a round and put on the safety without running the risk the that a M1911 may be dropped on its muzzle leading to an accidental discharge.
From the XM9 trial
The only deficiency noted for the 45 caliber pistol was safety It may, the JSOR states, accidentally discharge when the operator is attempting
to uncock the weapon or when the weapon 1sdropped. Army systems analysts, however, noted that they had not discovered any historical
data to support these safety deficiencies. In fact, theu- data base, assem- bled from 8 years of liaison visits to troop umts worldwide, contains no record of any problems of inadvertent discharge