People stopped buying sedans because the car industry made a concerted effort to demarket them. They've been pushing SUVs for nearly 30 years, trying really hard to get people to stop buying sedans and were too successful. Now there's a fuel crisis and no sedans in production (from American brands at least) so the manufacturers are getting caught with their pants down again

I do wonder if the fleet market is sufficient to keep both sedans and vans in production, since each makes a very competent fleet vehicle platform for different usecases, see the venerable Crown Victoria for example