I think you might find that the larger metropolitan areas are as you say. As one gets away from the coasts and into more rural areas you'll find smaller museums. Museums that are very bespoke and/or focused on a narrower curation target, etc.
I went to one in a tiny town in southern Arkansas a few years ago, dedicated to a river boat disaster shortly after the civil war. It was tiny, weird, and brilliant.
* Museum of the Fur Trade near Chadron, Nebraska * The Santa Fe Trail Center near Larned, Kansas * Ash Fork Route 66 Museum in Ash Fork, Arizona * National Mining Hall of Fame and Museum in Leadville, Colorado * American Windmill Museum in Lubbock, Texas * Strategic Air Command & Aerospace Museum near Ashland, Nebraska
The rural US has many local museums and most are surprisingly well done and very informative.