Holsters with covered or enclosed trigger guards were pretty rare prior to about 1990. Before that most holsters featured exposed trigger guard areas.
Why did this change? Primarily because of the introduction of striker-fired semi-auto pistols, many with no mechanical safety (Glock and others). Folks started carrying those and handling them as they were accustomed to with the earlier designs, and the results included pistols discharging in the holsters when contacted by seat belts, automobile upholstery, eager user's fingers going into the trigger guard during the draw, and other such events. Consequently a general change took place, with many law enforcement agencies and ranges (public and private) requiring covered trigger guards. Since then many people have become convinced that this feature is a requirement for any holster or handgun.
With double-action revolvers such as the S&W, Colt, Ruger, and most other modern designs pulling the trigger in DA mode requires about 20 lbs. of pressure applied over about an inch of travel, and the trigger must be held down while the hammer falls in order for the firing pin to strike a primer. The internal transfer bar will prevent firing, even if the revolver is cocked in single-action mode and the trigger is bumped rather than pulled and held down as the hammer falls.
I hear from a lot of younger shooters who have come to this activity within the past 20 years or so, anxiously wanting to know why I offer such "unsafe" holster designs as the Classic Tom Threepersons design (now 95 years old, and a standard for law enforcement and sportsmen for most of the 20th Century). Some are practically dribbling in their jeans just looking at a photo of such a horrible thing!
I'd like to see some of their faces if they ever saw the clamshell holsters used by some police departments, on which the trigger finger had to be pushed through the trigger guard to release the revolver from the holster!
For your Glocks, Kahrs, Springfield XD series, S&W M&P line, and some other recent arrivals a covered trigger guard is the best choice. For most of the older handgun designs (especially the modern double-action revolvers) it is purely a matter of personal preference (or occasionally a requirement of the insurance company covering your range).
