Why do think drive-in restaurants and theaters were so popular?

Mostly because they were associated with the car at a time when the car was a significant status symbol.Drive-ins came about soon after World War II, in the era of post-war prosperity that saw more and more people able to buy and drive a car. The car then, and even to a lesser degree today, had a allure of freedom about it, so people wanted to be able to go placed in it. Drive-in diners and theaters produced the novelty of letting people eat and watch movies without having to get out of the car. As cars became more commonplace (and thus began the headaches of traffic), the allure and novelty began to fade, though drive-ins still exist here and there (perhaps most notably in the Sonic chain of restaurants).