Hard to smile when you had to sit motionless for an hour while the image burned into the film back then.

That stopped being an issue very quickly.

Also, in the beginning it was not unusual that the only portrait of a person was post-mortem - to preserve the memory (as in 100 years of solitude). Difficult to smile in that case.

However, photographs were serious business for quite a while. You don't see people smiling on official portraits for the same reason. Imagine that portrait of Napoleon on a horse, but with a smile - doesn't have the same ring to it, does it?

Same for early photographs; they were expected to be solemn.