In the U.S., the answer usually comes back to racism. In the post-WWII era, a lot of mostly-white people moved to the suburbs. This pulled a lot of tax base and business away and also lead to a lot of neighborhoods being partitioned by highways so the suburban office workers could commute faster. In some areas, that combined with politics problems lead to riots in the 60s which further damaged many neighborhoods. All of that lead businesses not to invest or to pull out of less profitable neighborhoods. People who could afford cars would accelerate the shift by driving to the higher-end markets, so this can produce a negative feedback loop over decades, especially when businesses aren’t jumping to put money into remodeling or upgrading those locations.

Every time I’ve been in Europe I keep asking why we can’t have those markets, too. Trying to minimize time spent out of our cars and avoid contact with our neighbors has had a really big price.