Part of the problem is that they're seasonal in a lot of parts of the country. Who wants to go to a drive-in in the North in the middle of winter? It also occurs to me that cars aren't as convenient for this as they used to be - larger cars, low bench seats up front so you could get several kids up there, plus the people in the back could see over better, more convertibles, etc.
At the same time, I'd love to see them become more popular again in places with a lot of seasonal visitors, etc. Why? Because people can talk to each other as much as they *(@@# want in their cars.
In the 1970's, there was a drive-in in my town in Rhode Island - the same family owned that, a cinema in what had been a USO club (now gone), and another (single screen) in what had been an actual theater. No first-run movies - but you could go for a $1 to $1.50 depending on the night. The drive-in closed well ahead of the two theaters. Then again, the town still had two soda-fountain drugstores in the mid-70's, so it had a certain "time capsule" feel to it.
As a side note, whenever I hear about drive-ins, I always think about this O. Winston Link photograph: http://www.linkmuseum.org/images/collex/NW-1103.jpg
(I'll also put a plug in to visit the Link Museum if you're ever near Roanoke, VA - it, and the Virginia Museum of Transportation - are great)