Answer:
Transportation improvements
Transportation improvements, specially the automobile, allowed people to live further from their workplace. People took this advantage and moved to the surburbs, where they could inhabit quiet neighborhoods with more safety and greenery.
Commercial development
Commercial development in the suburbs, in the form of malls, strip malls, and big box stores, attracted people to live in these areas, because new comers could find all the goods and services that they needed in the immediacy of their homes.
