The Walther P38 started the trend in slide mounted decockers. The Army Pistol Trials of 1954 were being conducted, and with a leadership change at S&W, they jumped in using that design feature and others for their submittal which required 9mm, a 4" barrel, and lighter weight. That set the stage to a newer Army pistol, but It was decided that with 2.5 million 1911's in inventory, the Army didn't need to spend the money quite yet.
Subsequent to the Trials being discontinued, S&W released the M39 to the public. Beretta didn't release the 92 for another 20 years, about the same time S&W offered the double stack M59. Since the 3900 series had been on the market since 1955 I find it interesting to say Beretta set the standard.
The Illionois State Highway Patrol got the nation moving to auto pistols, and S&W was the lead company with the most sold for the next 20 years, finally eclipsed by Glock. Price competition was a major issue, plus the SAFE action trigger which simulates what a DA/SA does in some respects.
DA/SA has the advantage of being carried loaded in the chamber but with the longer trigger pull of a revolver with no required safety being on. When fired, they revert to SA and S&W's are well known for having a good trigger with short reset. Since the guns were selling well to police departments, the extra length of pull and higher trigger weight in the first shot DA mode was considered an advantage as an officer being suddenly stressed in a lethal encounter might be less likely to discharge the weapon prematurely. A lot of European military and police pistols share that feature for the same reason.
Some have issue with the two different trigger pulls and say it makes the pistol harder to shoot accurately. That case can be made by pointing to the more unlimited classes and their use of the 1911 in SA. However, that isn't necessarily a valid judgement as it could also be said a supercharged race car is much the same. CCW who carry (and drive) in everyday traffic might say maybe some power and air are a better idea. A pistol that has a smooth DA first shot isn't unpopular in America - most of the better pocket pistols and the line of Kahr self defense pistols use it, along with a lot of revolvers. Being able to have the gun switch to SA on the second shot does allow faster and more accurate follow up shots. In some trials a SA with safety and DA/SA were timed with a minimum number of shots performed and quite surprisingly the DA/SA was faster.
DA/SA is considered the more sophisticated design for professional carriers who have the gun on them daily. It's definitely arguable for some who would deny it but the more important point is that DA/SA has had a significant portion of the market for quite some time. It's not always about race guns and being able to carry loaded - but not cocked and locked - has a place in carry. It is exactly what we used to do with revolvers for decades and it works quite well.
