Indianapolis, That's Why.
When I was in Indianapolis a couple of years ago to speak at an Indiana Landmarks event, I was invited on a walking tour of an enclave of midcentury houses.
The state of preservation - and the quality of remodeling that had been done in cases where that was needed - was pretty amazing. Indianapolis is a very heritage-conscious town; there are many eras represented there, and they co-exist peacefully together, each well-preserved.
People there take care of what they've got. It was very nice to see.
P.S.: The house is not as unmolested as all that - there are Avocado majors in the kitchen, albeit well-installed and maintained. I'd also be willing to bet that the carpeting is hiding terrazzo flooring.