The price of gas wasn't the problem, it was the rising popularity of compact cars. They get great gas mileage, but are a little cramped for a four-hour double feature. One of the many drive-ins that now sit empty, a reminder of the glory days that are no more. One of the great things about a drive-in is that people have some privacy. They can talk during the movie if they want.
Kids can go in their pajamas and fall asleep in the back seat.
But the VCR meant people could watch movies in the privacy of their own homes. Even the adults can wear their pajamas. Although the number of drive-ins declined sharply in the 1970s and early '80s, the decline has leveled off. The remaining drive-ins are going strong. A few new drive-ins have even been built in recent years.