Earlier quoted context omitted.
Properly exposing the plate—or whatever—took several seconds, and it's way easier to look dour for five seconds than to hold a rictus grin. Source: Among other things, I've sat for a tintype portrait before. https://twitter.com/aaronbrethorst/photo
Tintype is a different technology. By the 1890s, it would have been as obsolete as rotary landline phones are today. I think the reason is different, because even I, as a kid in the early 1980s, was taken to a photo shop for "official shots", well dressed and told to behave. It was a bit unnatural, yes. People in staged shots are generally a little stiff.
However, you're incorrect about the lack of usage in the timeframe specified: "Tintypes enjoyed their widest use during the 1860s and 1870s, but lesser use of the medium persisted into the early 20th century" https://en.wikipedia.org/wiki/Tintype
Besides, the GP comment referred to the era, to which I was responding.
Your explanation is not without merit, however: "One common explanation for the lack of smiles in old photos is that long exposure times — the time a camera needs to take a picture — made it important for the subject of a picture to stay as still as possible. That way, the picture wouldn't look blurry." — https://www.vox.com/2015/4/8/8365997/smile-old-photographs