I always thought people didn't used to smile for photos because a photo was an important event. It was a serious, rare and expensive proposition. This article makes the case that people didn't smile for photos - or even for portraits - because only goofballs smiled. They said it more elegantly than I just did:
“it was a well-established fact that the only people who smiled broadly, in life and in art, were the poor, the lewd, the drunk, the innocent, and the entertainment.”
After all, as one French writer says, Nature gives us lips to conceal our teeth.
So remember that the next time somebody says "Cheese." A smile used to mean you were drunk.