A) Exposure times were very long in comparison to today's cameras and subjects were encouraged to sit very still for long periods of time. This might account for the fixed expression you refer to.
B) Having a photograph taken was a serious (and probably expensive) business.. not an occasion for levity ;)
They would, in the earliest photos, have to sit or stand in the same position for quite a long time in order for the photographic plate to be exposed to the light. The head and neck would often have been supported in a brace to keep it still.
Would you want to smile in those conditions?
On the other hand, it would have been a very serious business and a very prestigious thing to do. You'd have wanted to look important and wealthy, and important, wealthy victorians didn't smile in portraits.
yes, it was the equivalent of having your portrait painted; and most paintings didn't show people smiling much. (Laughing Cavalier and maybe Mona Lisa being exceptions). It was a costly business, and often undertaken to show the world just what a serious and important person you were.
The reasons photos were taken is also relevent, quite often it coincided with a formal event which would have taken place in a Church, e.g a wedding, Baptism or even a funeral. You will probably notice that most subjects appear in their 'Sunday best' which suggests there had been a special event to commemorate, this is why it is interesting to try to date photos to certain events in the family history.
Heard recently on one of those antique valuation TV shows that a high percentage of Victorians (except the very rich) had very bad teeth, so didn't want them painted.