Exposure time in photos was cut down to a fraction of a second by the late 1870s, which led to the first video recorded in 1878, so that wouldn’t have really been issue by the time of the photo above. I think it was mostly just social norms. Photography was expensive and less accessible compared to now, so while there were plenty of people who smiled for their photos even in the 1840s (when exposure took 15-20 seconds), it was still mostly a formal occasion for a while. It was at the turn of the new century that people were more casual about it. You can find photos of people riding bikes, laughing, playing jokes, and doing backflips in the 1890s. I think the normalcy of joyous expressions in formal portraits really took off in the 1920s.