According to this talk [0] the reason is that the newly established car industry encouraged a massively unsustainable development pattern that eventually led to the bankruptcy of the city and since its the first city to be built with this pattern it was also the first to fail. Lots of other cities are suffering the same fate. New low density development at the edge of town at the expense of the self sustaining downtown core and then at some point the maintenance costs exceed the tax income and you get a city wide decline.

[0] https://youtu.be/Em7nqDqQ8oM