How come large super market chains (Food Stores) won't stay in Detroit?

Detroit: 139 Sq Miles: 850,000 People. There are now no major chains in Detroit. The last (Farmers Jacks) is moving out. Major grocery chains, which generally operate with thin profit margins, say doing business in Detroit is no-win situation. High employee turnover, cost of security and loss from theft are often cited. The city's comparably low income rates preclude selling an abundance of high-profit, upscale items.