What happened is transportation changed. It used to be you had to live within walking distance of the factory. Now you didn't need to anymore. You could live in the suburbs and take the train to the city. So people did. No one wants to live so close to other people.
There's no reason to raise a family in the city. Our ancient ancestors understood this well. Send young adults to the city to make a fortune, find a spouse, and get out.
What happened is transportation changed. It used to be you had to live within walking distance of the factory. Now you didn't need to anymore. You could live in the suburbs and take the train to the city. So people did. No one wants to live so close to other people.
There's no reason to raise a family in the city. Our ancient ancestors understood this well. Send young adults to the city to make a fortune, find a spouse, and get out.