It's the action. In self protection, such as police work, they generally want an action that is ready to fire but also safe when holstered. A double-action revolver has historically been that weapon. A Glock acts like a double action revolver and requires no safety AND the second pull is very similiar to the first.
In contrast, a Beretta 92FS cannot be safely carried with the hammer back and the second pull is significantly different than the first. First double action shot is a long trigger pull, subsequent single action is a short pull. (I like the 92FS and have one myself). This difference affects the accuracy between first and second shots.
The 1911 is typically carried "locked and cocked" which means it is ready for single action mode with equivalent pulls for each shot but the safety must be disengaged for the first shot. Some police departments (actually most now) see this as a distraction in an emergency situation and don't want to deal with the safety. Most competition shooter prefer 1911's because of it's precise single action trigger and the safety is handled through training and repitition.
So basically Glock and the similiar variants from Sig and Kahr have an action that has similiar trigger pulls for the first and subsequent shots and can be carried safely in a "ready to shoot" mode without a manual safety. That's my understanding anyway.