Answer: Status dictated it
Step-by-step explanation:
In the past (and even now), middle class families often strove to be seen as people of higher status in the society much like the upper classes. For this reason they engaged in practices that were meant to show others that they were deserving of having status.
One of those practices was ensuring that the wife worked as little as possible outside the house. Women working was looked down on in the past especially for married women. It gave the impression of the husband being unable to adequately provide for his family and made people look down on the woman.
A middle class wife therefore, was expected to stay home, cook and take care of her husband and her children, supervise the home and ensure the man had peace of mind outside of work. This is what most of them did and why it was therefore uncommon to see a middle class woman working.