It's a common observation that people in old photographs aren't smiling. And I've often heard that the story about "it took 30 mins to take a picture" is an oversimplification of the reason why no one was smiling. But somewhere between daguerreotypes and polaroids, everyone decided that smiling was to be the done thing. How come?
You're right, long exposures are an oversimplification of a true answer. Cameras in the 1840s and 1850s had much longer exposure times than did cameras later on, sometimes lasting 15 minutes. By the 1860s and 1870s, these exposure times were usually only a few seconds; yet people really aren't smiling in those decades either.
A better reason to explain this is that photograph portraits were pretty much a faster and cheaper version of painted portraits. They were still thought of as formal and therefore, the pose and manner of a photo was formal too, which meant no smiling. But even though photos were cheaper than painted portraits, they still weren't exactly cheap, which means they weren't exactly everyday things. As time went on, prices went down and popularity went up. The cheapest camera you could buy out of the 1902 Sears and Roebuck catalog that I found was $1.68, and the average farmer in that time is making about $1 per day, with upper-middle class families usually making closer to $3-5 a day.
Since cameras were becoming more affordable and popular around the turn of the century, photographs became more casual because they were more common. That's where we see the change in everyday photographs. However, there's still plenty of "formal" portraits well into the 1900s where those in them still aren't always smiling.
I work at a living history museum primarily set in the Victorian era, so I hear questions and answers about a lot of different subjects and do a lot of research on the side, so that's where my information is coming from. Also I'm new to reddit, so this is actually the first time I've ever commented :)
FYI, there have been a few threads on this that you may find interesting:
When Did People Start Smiling in Photographs? featuring /u/Axon350
When did smiling in photographs become common? - more from /u/Axon350
When showing teeth (in art, photograph and portraits) became culturally acceptable? And why it wasn't before? - /u/GeeJo gives a quick survey of smiling in art
These threads are all archived by now, so if you have follow-up questions for any of these users, just ask them here and tag their username