I have heard a couple of explanations for this -
1 - It was a male oriented society. The head of the household sat, others stood and children sat on the floor.
2 - The style and design of the dresses. They were just not made for sitting in as they spread out too much.
3 - Lastly (and rudely) The picture was taken on the morning after the wedding night - He couldn't stand up and she couldn't sit down.
All my family studio photographs from that era are of the same format.
Regards
Chas
