One simple reason. Dead primers. A dead primer in a semi takes 3-5 seconds to correct. Less than a second in a revolver. Also, I have had a fair number of semi Auto firearms, pistol, rifle, and shotgun, with some very high end guns in the mix. Every single one has had a failure of some sorts at least once. I have never had a revolver malfunction. A glock may only jam 1 out of 1000, but that's just too risky when it comes to my wife and small sons.