Not because nobody wants to go, or because they don't pull in enough money to stay open, but because they are now essentially obsolete to the studios.
Spelled out in this Time article, movie studios will no longer print 35mm film sometime this year. This wouldn't normally be an issue, but it costs $70,000 to upgrade a screen to a digital projector. Most drive-ins simply do not pull in enough cash to justify that expenditure and for many it's easier to sell the land to someone who wants to develop it and walk away.
So, go out and support your drive-ins this summer. Buy the pizzas, the sodas, the ice cream, the popcorn. Let them know they still have your support and just maybe they'll be able to survive this latest test for our drive-ins.