Oftentimes, in the United States, when a city is prosperous, the middle class will start to want to move out of the city proper because it's too congested, they want a safer environment, don't mind the commute, et cetera et cetera. We'll call this the first ring of suburbs. Then, the first ring of suburbs start to be as congested as the city, so they move just a little further out. By the time they get to a third ring, the businesses have decided that it's a better idea to be where the people are, so they start moving to the third ring, vacating the city proper. By the time people are living in the sixth ring, the city starts to become a deserted hellhole and needs to find a way to reinvent itself to pull people back in.
Detroit's starting to do that, but due to a number of other factors including but not limited to Kwame Kilpatrick stealing millions of dollars from the city and short sighted negotiations on contracts (a lot of those deals were extremely backloaded because the government side wanted to punt paying money into the future without thinking that perhaps the future would have less revenue streams than the present).
I have other opinions on it that I may go into later, but these are a few reasons.