Essentially, before the 1880s or so, getting a photo taken was an experience. Most people didn't have their own camera equipment, because you needed all sorts of things like lenses, camera stands, plate holders, not to mention the whole darkroom setup. For this reason, when you got a photo taken it was because you wanted to have a record of how you looked. For decades, people had paintings or sketches for this reason, and to appear dignified and 'classy', they didn't smile. Smiling was seen as a strange expression to have in a painting.

The other reason has to do with technology. There are a few examples of 'instantaneous' photos taken before 1880, but they are few and far between, restricted to a few studios or experimental photographers that specialized in short exposure times. These experiments pale in comparison to the huge number of photos that required exposures of two to five seconds. That's out of the snapshot range; a smile is a fleeting expression that becomes uncomfortable after a second or two. Therefore, the photos that exist with smiles in them are because the photographer deliberately wanted to capture a subject's smile, not just his likeness.

It was when the photo was given to the masses that the snapshot became commonplace, and photography became about capturing a moment in time as opposed to an image of a person. The Kodak in 1888 was released with a shutter speed of 1/25 of a second, enough to snap a picture of a grin. Men photographed their smiling wives, mothers photographed their happy children, and the casual market for photography grew huge.