The lack of smiling in early pictures is due to two factors: pictures took a long time to take, and many pictures were formal portraits. Because early film took a long time to capture the picture, the subject or subjects had to remain almost completely still to show up. There's a picture of a street in broad daylight where the street has no people except one man who was having his shoes shined, so he stayed still long enough to show up in the picture, that illustrates this. Because a person had to stay still, it was more comfortable to maintain a neutral facial expression than to smile. Second, many single-subject pictures were portraits, done in much the same style as painted portraits. Pictures were expensive and not accessible, much like painted portraits. So, to answer your question, it was when pictures, and, by extension, cameras, became cheaper and less time-consuming to take.