A lot of people left over the years. That works out to empty space and empty buildings - what really did you expect?
The real low point was the late 80s. At that point, the city had gone decades with little or no new development as many waited to see when, or if, the tide would turn. No building permits were issued in 1988, a time when the city was the country’s 7th largest by population (it would later fall to 18th). Between 1978 and 1998, the city issued 9000 building permits and 108,000 demolition permits, and in 1990, the City spent twenty five million dollars on the removal of abandoned property. Those are mind boggling numbers for a major city.
Today, downtown and a few other pockets are doing better, and while the city's finances have a ways to go... it's attracting young people - the only population segment to grow 2000-2010. Parts of corktown might as well be Brooklyn. Whole foods is moving in. There's still a lot of empty space...but that means opportunities for the folks who couldn't afford a regular, dense urban environment.
For young entrepreneurs who don't mind a little urban frontierism...it's a pretty appealing place.