Off the top of my head, since you're talking about posed photos, I'd say it could have been a hang-over from when it was necessary to hold a pose for a few seconds due to longer exposure times. The idea of a posed (professional?) photo being serious may have been current, and if it cost a fair bit of money some people wouldn't have been laughing about it. What changed? When did cheap cameras and processing become widely available and used? I'd guess that once people were more used to having their picture taken in an informal setting, the idea of smiling in a posed photo became more acceptable.
Do you have more details of the sorts of photos you've been looking at?