Some of the earliest drive-in theaters date back to the early 1900s, but their true reign in American culture peaked from the 1940s through the 1960s. A drive-in movie theater consists of a large parking lot or field, a large outdoor screen or projector, and some form of concessions stand. Drive-ins provided a unique movie-watching experience from the comfort of one’s car, and allowed for ultimate privacy.
The true push for drive-in movie theaters came from the boom of car sales after wartime. The cost to maintain such a property was less than the cost to run an indoor movie theater, which, in turn, allowed for ticket prices to be greatly cheaper than the alternative, thus drawing younger audiences and families.
Source: Explained: Why the Drive-In Is An Important Part of American Cinema History