Answer:
Availablity of cars, a growing middle class, and the GI Bill.
Step-by-step explanation:
Levittowns were suburban communities developed in the 1950's within the US. These are named after William Levitt, a military man who used his knowledge of construction in order to make neighborhoods of nearly identical houses. Due to his knowledge of construction, Levitt was able to produce these houses cheaply and inefficiently.
These Levittowns were able to grow thanks in large part to the GI Bill. This law helped WWII veterans to afford homes by offering low interest home loans. Along with this, the ability of Americans to afford cars gave them the opportunity to live outside of the city.
Lastly, the economic prosperity of the 1950's and the increase in disposable income allowed for the emergence of a new middle class that could afford to buy these homes in the suburbs.