Think about it from the perspective of someone who lived around that time. The family portrait was previously something that took hours and hours of an artist working away while you and your family just sat there,.
It was HIGHLY cost prohibitive and a symbol of serious wealth.
the photograph made this process easier, faster, and cheaper. So the lower income families who could suddenly afford it emulated the style of the family portrait. Also while it was cheaper, it was still really expensive and so the whole family probably only took one.