Two, zoos aren't like businesses or services; they don't naturally appear in an area just because the area is capable of supporting one. Each zoo is a unique institution, with its own backstory of how it started and how it has remained intact over the years.
I have seen the other thread as well. But your second point hits it on the nose for me. The backstory and history are definitely the reason many zoos that are around now days are still open. Especially, a zoo like Chahinkapa Park and Roosevelt Park in North Dakota and others like even Comp Zoo in St. Paul.